Method and apparatus for distinguishing synthetic diamonds from natural diamonds

A method and apparatus are provided for distinguishing synthetic diamonds from natural diamonds by detecting the effect the synthetic diamonds have on a magnetic field. A counter can be provided for measuring counting transitions of an oscillator having an inductor in a frequency determining circuit. The inductor has a port therein for admitting a sample diamond for testing. First, the oscillations are up counted over a precise time interval with no sample present, and then a sample diamond is placed into coupled proximity with the inductor to change the frequency of the circuit by an amount related to the quantity of ferromagnetic material in the sample diamond. Thereafter, a down count is performed for the oscillations with the sample diamond in place over a second time interval equal to the first interval. The difference in the up counts and the down counts is determined and displayed in a digital readout as a measure of the degree of likelihood of the sample being synthetic. Alternatively, a sample diamond can be placed in coupled proximity to a coil in a tuned circuit. A resultant change in the output amplitude of the tuned circuit is detected from which a display is generated indicating the degree of likelihood of the sample being synthetic. A sample diamond can also be rotated in a magnetic field. A signal induced in a coil is detected from which an indication of the degree of likelihood of the sample being synthetic is displayed.

FIELD OF THE INVENTION
 This invention relates to the screening of diamond gemstones so as to
 distinguish and detect synthetic diamonds from natural diamonds. More
 particularly, the invention relates to a method and apparatus for the
 rapid screening of diamond gemstones to determine the need for further
 investigation by identifying those likely to be synthetic. The invention
 produces a negative response for natural diamonds but reacts to the
 magnetic properties of synthetic diamonds to provide an indicator thereof.
 BACKGROUND OF THE INVENTION
 With the advent of gemstone quality synthetic diamonds the need has arisen
 for a rapid screening system available and affordable for the local
 jeweler to establish with reasonable probability whether a given diamond
 gemstone is natural or synthetic.
 Existing systems for detection of synthetic diamond gemstones are
 summarized in the article Shigley et al., "Sumitomo Synthetic Diamonds",
 Gems and Gemology, Winter 1986, pages 192-208. As there indicated the
 various properties of diamonds and particularly those that distinguish
 natural from synthetic stones are compared. Testing procedures were set
 forth which included color, (which may involve spectroscopy examination),
 fluorescence, electrical conductivity, thermal conductivity, specific
 gravity, microscope inspection, reaction to polarized light, and
 magnetism. As to the latter, magnetism, the test procedure involved gross
 attraction of the tested diamond to a magnet. However, it was found that
 only one of the tested synthetic diamonds was attracted so that the
 reliability of a test as there proposed was found inadequate.
 A subsequently published article by Shigley et. al., "Gemological
 Properties of the De Beers Gem Quality Synthetic Diamonds", Gems and
 Gemology, Winter 1987, pages 187-206, repeated the previous work and
 investigation as applied to the De Beers synthetic diamond product. In
 addition to the tests earlier performed, the last publication also
 indicates that a test was conducted with catholuminescence and specific
 gravity investigations as well as chemical analysis of inclusions. As to
 the magnetic behavior it was noted that natural diamonds are only weakly
 magnetic if at all and that synthetic diamonds were believed to vary from
 strongly magnetic to non-magnetic. Accordingly, these studies concluded
 that magnetic investigation of synthetic diamond gemstones was not useful
 for identification. They further state that they foresaw difficulties in
 separating natural from synthetic colorless diamonds using any other
 conventional gemological technique.
 In addition, various techniques have been used to determine the content of
 magnetic compounds such as magnetite and pyrhotite in ore samples. For
 example, U.S. Pat. No. 3,808,524 (Tarassoff et al., "APATUS FOR
 DETERMINING THE AMOUNT OF MAGNETIC MATERIAL IN A SAMPLE") discloses an
 apparatus that determines the amount of a magnetic compound in an ore
 sample by inserting the sample into a coil of an oscillator and detecting
 the change in the oscillating frequency of the oscillator.
 Conventional teachings in the synthetic diamond art indicate that synthetic
 diamond screening tests based on magnetic qualities of the diamonds are
 inadequate. Thus, these teachings suggest that methods such as those
 disclosed in Tarassoff would probably be ineffective. Accordingly, a need
 exists for a relatively easy to use and inexpensive method and apparatus
 for distinguishing between synthetic diamonds and natural diamonds.
 SUMMARY OF THE INVENTION AND OBJECTS
 In general, it is an object of the present invention to provide a method
 and apparatus for distinguishing synthetic diamonds from natural diamonds
 which will overcome the above limitations and disadvantages.
 It is a further object of the invention to provide a method and apparatus
 of the above character based on a test for magnetic susceptibility of the
 stone.
 It is known that natural diamonds exhibit very little if any magnetic
 behavior which could form the basis of their evaluation. However,
 synthetic diamonds have been found to contain magnetic inclusions inherent
 from the process from which they are made. Even though these inclusions
 can be quite small they are nevertheless found to exist in all synthetic
 diamonds that have been investigated whether or not detectable by the
 gross magnetic attraction methods mentioned in the literature. Even the
 clearest of synthetic stones contain ferrous material presumably more
 evenly distributed at the molecular level.
 The present invention is based upon the realization that, contrary to
 conventional teachings in the synthetic diamond art, synthetic diamonds
 can be distinguished from natural diamonds by testing the magnetic
 properties of the diamonds. Specifically, trace amounts of iron are
 detected in synthetic diamonds using sufficiently sensitive instruments
 capable of detecting the effect the trace amounts of iron have on a
 magnetic field.
 In a first embodiment, the present invention is predicated upon the
 realization that, given a suitably sensitive and appropriate
 instrumentation, it is possible to screen synthetic diamond gemstone from
 natural diamond gemstones by placing the stone under test into the core of
 an inductance in a linear oscillator circuit in which the magnetic
 character of the synthetic diamond changes the inductance and affects the
 frequency of the oscillating circuit in a way that a difference count can
 be obtained over a precisely repeatable interval for the condition of no
 sample present, compared to that when the sample is present, by an
 extremely sensitive but stable counting circuit. It is found that a
 reliable screening method can be based on this concept.
 Generally, the invention provides a counter for measuring or counting
 transitions of an oscillator having an inductor in a frequency determining
 circuit. The inductor is in at least in part air cored, being wound around
 a core form having an opening therein for admitting a sample into the
 inductor core for testing. The method calls for upcounting the
 oscillations of the oscillator over a precisely repeatable time interval
 with no sample present and thereafter selectively placing an unknown
 sample diamond stone into coupled proximity or within the core of the
 inductor to change the oscillator frequency by an amount related to the
 ferromagnetic material contained therein. At that point the system is
 signaled to down-count the oscillations, subtracting the same from the
 up-count over a second time interval precisely equal to the first
 interval. The difference in counts is determined and displayed on a
 digital readout as a measure of the degree of likelihood that the sample
 under test is synthetic.
 In a second embodiment, the diamond sample is positioned near a coil of a
 tuned circuit. When the diamond sample is a synthetic diamond, the iron in
 the diamond sample alters the parameters of the coil (e.g., the "Q" of the
 coil) which, in turn, changes the amplitude of the output signal of the
 tuned circuit. The change in amplitude is detected and displayed to
 indicate the degree of likelihood that the sample under test is synthetic.
 The second embodiment typically provides an even more sensitive screening
 device than the first embodiment. First, the second embodiment uses a
 fixed frequency oscillator. As a result, its sensitivity is not affected
 by frequency drift as much as the first embodiment. Second, using tuned
 circuits and a differential amplifier, this embodiment can detect very
 small changes in the Q of the coil. Consequently, this embodiment provides
 a very sensitive screening apparatus.
 In a third embodiment, the diamond sample is moved within a magnetic field.
 When the diamond sample is a synthetic diamond, iron inclusions within the
 diamond disturb the magnetic field. This disturbance generates a signal
 that is detected by the screening apparatus. This signal is amplified and
 processed to generate a display indicating the degree of likelihood that
 the sample under test is synthetic.
 Typically, this embodiment provides an even more sensitive screening device
 than the embodiments discussed above. Consequently, this embodiment
 provides a very effective yet relatively simple screening device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Referring to FIG. 11, a block diagram illustrates a synthetic diamond
 screening device S according to the present invention. The screening
 device S identifies a synthetic diamond by detecting magnetically
 sensitive inclusions in the synthetic diamond. The inclusions result from
 the process of producing a synthetic diamond by growing synthetic diamond
 crystals in an iron-based flux. The presence of the flux causes iron
 particles to become entrapped in the crystal during the growth process.
 The screening device S detects inclusions in a diamond sample by detecting
 the effect the inclusions have on a magnetic field. A magnetic field
 generator 50 (FIG. 11, left) produces a magnetic field (not shown) and a
 diamond sample support 52 positions a diamond sample D in the magnetic
 field. A diamond sample magnetic field alteration detector 54 detects the
 effect the diamond sample D has on the magnetic field and a synthetic
 diamond decision circuit 56 generates a signal which indicates the
 probability that the diamond sample D is synthetic. An output signal
 generator 58 processes this signal to generate an output signal that
 drives an output device 60. The output device 60 provides a visual
 indication of whether the diamond sample D is a synthetic diamond or a
 natural diamond.
 Three embodiments of the present invention are presented. In a first
 embodiment, a diamond sample is placed in a magnetic field generated by a
 coil of an oscillator. The presence of ferrous material (i.e., iron) in
 the diamond sample changes the effective inductance of the coil which, in
 turn, changes the operating frequency of the oscillator. The apparatus
 determines the probability that the sample is synthetic based on the
 change in the operating frequency. In a second embodiment, a diamond
 sample is placed in a magnetic field generated by a coil of a tuned
 circuit. In this embodiment, a change in the Q of the coil changes the
 amplitude of the output signal of the tuned circuit. The apparatus
 determines the probability that the sample is synthetic based on this
 change in amplitude. In a third embodiment, a diamond sample is rotated in
 a magnetic field. The presence of ferrous material in the diamond sample
 disturbs the magnetic field and generates a signal in a magnetic head. The
 apparatus determines the probability that the sample is synthetic based on
 this signal.
 Referring to FIG. 1, the first embodiment of the present invention screens
 a diamond sample by detecting a change in the operating frequency of an
 oscillator that results from placing the sample in the frequency
 determining coil of the oscillator. A sample oscillator 15 and its
 associated coil 10 substantially provide the functionality of the magnetic
 field alteration detector 54 of FIG. 11. The remaining components of FIG.
 1 substantially provide the functionality of the signal processor 62 and
 the output device 60 of FIG. 11.
 A sample receiving coil 10 mounted with one end 12 opening through the top
 of a case into which the operator places the stone 14 to be tested. Coil
 10 forms an inductance which is part of a frequency determining circuit
 configuration of a free-running sample oscillator 15. Current flowing
 through the coil 10 generates a magnetic field around the coil 10. Iron in
 the stone 14 interacts with the magnetic field and changes the inductance
 of the inductive circuit which includes the coil 10 and the stone 14.
 Because the inductive circuit is part of the frequency determining circuit
 of the oscillator, the change in the inductance of the inductive circuit
 changes the operating frequency of the oscillator.
 The apparatus of FIG. 1 processes the change in the operating frequency of
 the oscillator to provide a visual indication on a digital readout (FIG.
 1, right) of the probability that the stone 14 is a synthetic diamond. A
 stable reference oscillator 16 clocks a counter 18 and control logic 20
 controlled by a user calibrate/measure switch 22 to establish an up-count
 reference interval initiated by the operator before inserting the stone
 into coil 10. This interval is exactly matched after an unknown stone is
 placed for test and the operator gives the start test signal by releasing
 the switch 22.
 During each of the reference interval and the test interval, the number of
 transitions of the sample oscillator is determined, and the down-counts of
 the latter are subtracted from the up-counts of the former to obtain a
 difference count which is displayed in a suitable readout. When the stone
 is natural, it contains no ferrous material, and the up-counts and
 down-counts match so that the output displayed is very low or zero. But,
 when the stone contains ferrous material, either in inclusions or
 distributed at the molecular level, the frequency of the sample oscillator
 is changed, so that the down-count will not equal the up-count. The
 difference of the counts is then displayed in digital readout 24 which
 signals the likelihood of a synthetic stone whenever the value exceeds a
 predetermined threshold level. The apparatus uses a stable voltage
 reference supplied by a regulated power supply 26 which can be connected
 to AC mains.
 Sample Oscillator Circuit 15
 Referring now to FIGS. 1 and 2, the sample oscillator is made up by coil 10
 represented in the circuit diagram as inductor L1, transistor Q1,
 resistors R15 and R16 and capacitors C8, C9, C10 and C11 connected in a
 Hartley configuration. C8 and L1 determine the frequency of oscillation of
 the oscillator circuit.
 Inductor L1 is of an open core form comprising a coil of wire wound on a
 hollow form 30 closed at the bottom, but open at the top to define a test
 chamber 31 into which the diamond to be tested is placed. The coil form
 opening 12 is circular, and of a diameter suitable for receiving a diamond
 of the size desired to test. A range of sizes can be tested with a form of
 a given opening. By way of example, an opening of from 7 to 9 millimeters
 can accommodate stones up to about 1.5 carats. The form opens upright so
 that the diamond stone to be tested may be dropped in from the top and
 removed by turning the tester case over. Alternatively, the stone can be
 temporarily attached to a strip of adhesive tape and lowered into the test
 chamber, or it can be inserted and removed with a pair of plastic
 tweezers.
 The coil length is about 3/10ths of an inch with the test chamber 31
 contained within the coil length. While it is preferred that the stone be
 fully placed within the test chamber for reproducibility of results, this
 is not critical. Nor is the portion of the stone within the coil and
 chamber; and, the stone may be placed either point end down or up.
 The frequency of oscillation of the sample oscillator 15 changes when a
 synthetic diamond containing a small amount of ferrous material (typically
 iron and/or nickel) is placed in the test chamber 31 of the coil. The
 frequency does not significantly change when a natural diamond without any
 ferrous material is placed in the coil.
 The output of the sample oscillator is isolated from the remainder of the
 detector circuits by integrated circuit U8C. This prevents the changing
 load at the OSC output from pulling the frequency of oscillation. The
 isolation circuit is made up of integrated circuit U8C, capacitor C14,
 resistors R11, R12 and R13. Diode CR1 prevents the input at integrated
 circuit U8C from going negative and being damaged.
 The frequency of the sample oscillator is approximately 800 Khz. The
 diamond detector will function properly with an oscillator frequency of
 100 Khz to 2 Mhz. The frequency can be changed by changing the value of C8
 or the number of turns of the coil 10 comprising L1. The output of the
 sample oscillator is a continuous sine wave signal appearing at the
 oscillator output 35.
 Reference Oscillator 16
 Referring now to FIGS. 1 and 3, the reference oscillator 16 provides a
 stable, accurate and repeatable time interval for the calibrate and
 measurement cycles. The calibration and measurement cycles time interval
 must be equal to a high degree of accuracy for the diamond detector of the
 invention to work properly.
 The reference oscillator 16 includes a crystal XL1, integrated circuit U6,
 resistors R14 and R17 and capacitors C12 and C13 which together form a
 stable crystal controlled oscillator. The frequency of oscillation is set
 by XL1.
 The integrated circuit U6 also contains a 21 state counter which divides
 the crystal oscillator circuit by 2,097,152. Integrated circuits U2A and
 U2B form two divide by 2 circuits.
 AND gate U13A gates the output of the three divider circuits to form the
 required stable, accurate and repeatable time intervals. See FIG. 4 for
 the timing diagram of the reference oscillator 16.
 The satisfactory time interval from the reference oscillator (at REF CLOCK)
 is approximately 4 seconds. The diamond test circuit will function
 properly with time intervals from approximately 1/2 second to 8 seconds.
 The time interval can be changed by changing the crystal XL1.
 Push to Test Switch Circuit 22
 Referring to FIGS. 1 and 5, the push to test circuit 22 includes a spring
 biased open switch S1 which controls the operation of the apparatus. When
 S1 is depressed, the output of integrated circuit U8A transitions to logic
 HI starting the up count or calibration cycle. When S1 is released the
 output of U8A transitions to logic LO starting the measurement cycle.
 Alternatively, S1 can be of a type that requires a second pressing to
 activate the second part of the cycle.
 Resistor R3 :Ls used to provide a logic HI at the (-) input to integrated
 circuit U8A when S1 is not depressed. Capacitor C6 along with the
 hysteresis circuit around integrated circuit U8A work together to
 eliminate S1 switch bounce when S1 is depressed or released. Resistors R4,
 R5 and R6 form the hysteresis circuit around U8A. Resistor R7 at the
 output of U8A is used to pull the output of U8A to logic HI when S1 is
 depressed, as U8A has an open collector output.
 Control Logic 20
 The control Logic is shown in FIGS. 1 and 6 and consists of three sections,
 as follows: a reset circuit, a count-up circuit, and a count-down circuit.
 The reset circuit consists of integrated circuits U8B, U14A, U1D, resistor
 R2 and capacitor C5. R2 and C5 set the pulse width of the RESET- and send
 the clear pulse to integrated circuits U3A and U7A. Whenever the count of
 the up/down counter equals zero or the push to test switch S1 is depressed
 (UP/DN--transitions to logic HI), U5A outputs the RESET--and the clear
 pulse.
 The count up circuit consist of integrated circuits U1C, U3A, U3B, U15A,
 U15C, U1A, U13B, U7A, LED CR2-A and resistor R19. Integrated circuits U3A
 and U7B are cleared when the push to test switch is depressed
 (UP/DN--transitions to Logic High). The green LED is turned off at this
 time. On the next high to low transition of the REF clock, the OSC output
 is gated to the Count Clock by integrated circuit U15C, U1A and U13B.
 Integrated circuit U3B is cleared at this time by integrated circuit U3A Q
 output starting the count up cycle. On the next low to high transition of
 the REF clock the OSC output is gated off at integrated circuit U13B
 output by integrated circuit U3B Q, U15C and U1A. At this time integrated
 circuit U3B Q output clocks integrated circuit U7A turning on the red LED
 CR2-A indicating the up count or calibrate cycle is complete.
 Count down circuit consist of integrated circuits U1C, U4A, U4B, U15B,
 U15D, U1B, U13B, U5B, U7B, LED CR2-B and resistor R18. Integrated circuits
 U4A and U7A are cleared when the push to test switch is released
 (UP/DN--transitions to logic L0). The red LED is turned off at this time.
 On the next HI to LO transition of the REF CLOCK the OSC OUTPUT is gated
 to the count clock by integrated circuit U15D, U1B and U13B. Integrated
 circuit U4B is cleared at this time by integrated circuit U4A Q output,
 starting the count down cycle. On the next LO to HI transition at the REF
 Clock the OSC output is gated off at integrated circuit U13B output by
 integrated circuit U4B Q, U15D and U1B. At this time integrated circuit
 U4B Q output clocks integrated circuit U7B turning on the green LED CR2-B
 indicating the count down or measurement cycle is complete. Also at this
 time, integrated circuit U5B--TR input is clocked causing a STORE--pulse.
 Resistor R1 and capacitor C4 set the STORE--pulse width.
 See FIGS. 7A-7I for control logic and system timing graphs.
 Up/Down Counter Display 24
 Integrated circuits (integrated circuit) U9 and U10 form an eight digit LED
 display driver with programmable up/down counter. This counter is
 controlled by the following four inputs and two sets of outputs
 "RESET-" This input when pulsed logic low will reset the counter to zero.
 "STORE-" This input when pulsed logic low will store the existing value of
 the counter and cause it to be displayed on the four digit display U11 and
 U12.
 "UP/DN-" This input when logic HI causes the counter to count up one count
 for each "COUNT CLOCK" cycle. When logic low the counter will count down.
 "COUNT CLOCK" Each cycle of the count clock causes the count to count up or
 down one count depending on the state of "UP/DN-".
 "COUNT EQUAL ZERO" This output goes logic HI when the count of both
 integrated circuit U9 and U10 equal zero.
 Display drive a,b,c,d,e,f,g,D1,D2,D3 and D4 drive LED display integrated
 circuit U11 and U12.
 Voltage Regulator 26
 The voltage regulator has two (2) +5vDC regulator circuits. One for the
 sample oscillator circuit and one for the remainder of the diamond test
 circuit. The separate voltage regulator for the sample oscillator improves
 the sample oscillators stability.
 Voltage regulator integrated circuit Q2 and capacitor C2 form the sample
 oscillator +5vAC supply. Capacitor C2 stores energy and reduces ripple at
 the output of Q2.
 Voltage regulator integrated circuit Q3 and capacitors C1 and C2 form the
 +5vDC supply for the remainder of the detectors circuits. Capacitor C1 and
 C2 store energy and reduce supply ripple.
 Resistor R20 output provides a logic HI pull up (PU) for the logic inputs
 that need to be a logic HI at all times. Resistor R8 and R9 form a
 resistive divider to produce a 2.5vDC reference voltage. Capacitor C7
 provides decoupling for the 2.5vDC reference. A +9vDC level is provided to
 the diamond detector by a standard UL approved regulated wall mount power
 supply.
 System Operation
 In summary, the diamond test has four operator interface indicators and
 controls. They are the push-to-test switch, 4 digit LED (should this not
 be liquid crystal) display, red and green LED indicators and the sample
 coil.
 The sequence of operation of the diamond detector is as follows (after
 plugging in the +9vDC wall mount power supply and allowing the detector
 approximately 5 minutes to warm-up):
 The push-to-test switch 22 is pressed FIG. 7B) and held down. The green LED
 CR2-A will go out (FIG. 7F). After approximately 6 seconds the red LED
 CR2-B will illuminate (FIG. 7E) indicating the completion of the
 calibration cycle in which the count clock counts up the number of sample
 oscillator cycles (FIG. 7C) during the calibration interval (FIG. 7A).
 2) The diamond to be tested is placed in the sample coil and the
 push-to-test switch is released FIG. 7B). The red LED will go out at this
 time FIG. 7E).
 3) During the next available interval (FIG. 7A) the count clock recounts as
 a down-count the sample oscillator cycles (FIG. 7C). After approximately 6
 seconds the green LED will illuminate (FIG. 7F) indicating completion of
 the measurement cycle and test. The STORE signal is given to the display
 circuits (FIG. 7H) and the results displayed (FIG. 7I). At this time the
 results of the test will be displayed on the four digit display.
 It is found that stable oscillators will result in count differences
 between natural and synthetic stones of above about 25 counts for signals
 having total count values of about 2 million. This leads to the
 requirement that the counting circuits be stable over the testing process
 time to less than that value, preferably to about one (1) part per million
 for 5-10 seconds. Stability is easily checked, by the way, by performing
 the entire sequence without a sample stone in place. This should result in
 a count of less than 3 cycles.
 As an example of the criteria for a synthetic stone identification in
 accordance with the present invention, for a sample oscillator frequency
 of 800 Khz, and a sampling interval of 4 seconds, it is expected that no
 natural stone will result in a count difference greater than 25 cycles.
 Any value greater than 25 cycles should be considered to be likely to have
 resulted from testing a synthetic diamond.
 While the invention has been disclosed in a form for ready implementation
 in discrete logic circuits, it is to be understood that the reference
 oscillator, control logic circuits and counting circuits could readily be
 implemented on a computer chip with appropriate programming.
 By way of an example, and referring to FIG. 10, there is shown a logic
 diagram representing the logic circuit program for a 20V8 PLD
 (programmable logic device) chip. Such a PLD chip is provided with an
 internal logic gate block for realizing a logic circuit specified by the
 user in the form of a circuit configuration data written into a memory
 circuit. The form of circuit configuration is the logic circuit itself
 such as is shown in FIG. 10. After receiving the logic circuit
 configuration, the PLD chip performs in accordance with that logic. When
 so programmed according to the logic of FIG. 10, the 20V8 PLD chip can
 directly replace the entire circuit of FIG. 6. The 20V8 PLD pin
 assignments are given as arrowed references 1-23 in FIG. 10. The
 equivalent connections for substituting the chip into the previously
 described circuit of FIGS. 1-9 are given immediately over the respective
 leads, as U3A,Q . . . U10,Ct. It is not necessary to set forth the
 programming in detail as the logic configuration shown is used directly
 for programming the PLD chip. Such logic chips are available from Advanced
 Micro Devices of Sunnyvale, Calif. and from Monolithic Memories of
 Sunnyvale, Calif., among others. By using a programmable logic device the
 size and cost of the circuits is reduced.
 Referring now to FIG. 12, the second embodiment of the present invention
 screens a diamond sample by detecting a change in the output amplitude of
 a tuned circuit that results from placing the sample in a coil of the
 tuned circuit. An oscillator 64 (FIG. 12, left) and a sample tuned circuit
 68 substantially provide the functionality of the magnetic field
 alteration detector 54 of FIG. 11. The remaining components of FIG. 12
 substantially provide the functionality of the signal processor 62 and the
 output device 60 of FIG. 11.
 The fixed frequency oscillator 64 generates a signal that drives two tuned
 circuits: a reference tuned circuit 66 and the sample tuned circuit 68.
 The two tuned circuits are identical except that the sample tuned circuit
 68 has a sample coil 70 adapted to receive a diamond sample D. The outputs
 of the two tuned circuits drive a comparator 72. When a diamond sample D
 is not present in the sample coil 70, the output signals from the two
 tuned circuits are virtually identical. As a result, the comparator 72
 does not generate an output signal. When a diamond sample D is placed in
 the coil, the Q of the sample coil 70 changes. This, in turn, changes the
 amplitude of the output signal of the sample tuned circuit. The comparator
 72 senses the difference in the outputs of the two tuned circuits and
 generates an output signal. The output of the comparator 72 is amplified,
 filtered and processed to generate a signal that drives a display device
 74. The display device 74 provides a visual indication of whether the
 diamond sample D is synthetic or natural.
 A detailed circuit diagram of the embodiment of FIG. 12 is depicted in
 FIGS. 13A, 13B, 13C and 13D. Referring to FIG. 13A, a crystal controlled
 oscillator circuit 64 (left) drives a reference tuned circuit 66 and a
 sample tuned circuit 68. The oscillator circuit 64 is comprised of crystal
 oscillator XL2, transistor Q4, capacitors C17, C18 and C19, and resistors
 R21 and R24. The output signal of transistor Q4 feeds capacitor C6 and a
 voltage divider comprised of resistors R22 and R23. This signal drives the
 reference tuned circuit 66 and the sample tuned circuit 68.
 As shown in FIG. 13A, the oscillator circuit 64 can be constructed from
 relatively inexpensive electronic components. For example, oscillator XL2
 is a fixed-frequency crystal oscillator. Typically, the oscillator has an
 operating frequency between 2 to 8 megahertz. Transistor Q4 is a low-cost
 general purpose transistor. The construction and operation of oscillator
 circuits such as the one depicted in FIG. 13A are well known in the
 electronics art. Accordingly, the oscillator circuit will not be discussed
 further.
 The reference tuned circuit 66 generates a reference signal that is
 compared by the comparator 72 (FIG. 13B) to a signal from the sample tuned
 circuit 68. The operation and construction of tuned circuits are well
 known in the electronics art. Accordingly the operation of the tuned
 circuits will be discussed in general terms.
 The frequency response of a tuned circuit describes the relationship
 between the frequency of the signal driving the tuned circuit and the
 output amplitude of the tuned circuit. Graphically, the frequency response
 resembles a "bell curve" where the output is maximum at the resonant
 frequency of the tuned circuit and is progressively lower at frequencies
 above and below the resonant frequency. The steepness of the bell curve
 near the resonant frequency is related to a characteristic of the tuned
 circuit known as the "Q" of the circuit. A tuned circuit with a high Q has
 a relatively steep curve near the resonant frequency. In other words,
 relatively small changes in the input frequency produce relatively large
 changes in the output amplitude of the tuned circuit. Similarly, the phase
 shift between the input and output signals of a high Q tuned circuit is
 relatively sensitive to a change in the input frequency when the input
 frequency is near resonance. In sum, a high Q tuned circuit is relatively
 sensitive to changes in the frequency of the input signal particularly
 when the frequency of the input signal is near the resonant frequency of
 the tuned circuit.
 In a similar manner, a high Q tuned circuit is relatively sensitive to
 changes in the inductive components of the tuned circuit when the input
 signal frequency is near resonance. A change in the inductance of a tuned
 circuit changes the Q and the resonant frequency of the tuned circuit.
 This causes the output amplitude and phase shift of the tuned circuit to
 change. Moreover, as discussed above, when the input frequency is near
 resonance, the tuned circuit is particularly sensitive to changes in the
 resonant frequency relative to the input frequency. As a result, when the
 input signal frequency is held constant at the resonant frequency, a
 relatively small change in the parameters (e.g., inductance or Q) of the
 tuned circuit produces a relatively large change in the output signal
 amplitude and phase shift of the tuned circuit.
 The reference tuned circuit 66 of FIG. 13A includes operational amplifier
 U17, capacitor C21 and inductor L3. The operational amplifier U17 along
 with the operational amplifier U16 in the sample tuned circuit 68 provide
 isolation between the tuned circuits and the signal from the oscillator
 circuit 64. This isolation prevents a change in one of the tuned circuits
 from affecting the signal that drives the tuned circuits. The capacitor
 C21 and inductor L3 in the reference tuned circuit 66 are the capacitive
 and inductive elements for the tuned circuit, respectively. Thus, the
 values for capacitor C21 and inductor L3 are selected to make the resonant
 frequency of the tuned circuit approximately equal to the frequency of the
 oscillator circuit 64 and to give the tuned circuit a relatively high Q.
 For example, typical component values when the oscillator frequency is
 approximately 2.4 megahertz are 16 microhenries for inductor L3 and 270
 picofarads for capacitor C21.
 The sample tuned circuit 68 is identical to the reference tuned circuit 66
 except that the sample coil 70 (i.e., inductor L2) is adapted to receive a
 diamond sample D within its core. When a diamond sample D is not in sample
 coil 70 (as represented by the diamond sample D with solid lines), the
 outputs of the two tuned circuits are virtually identical. However, when a
 synthetic diamond is placed in sample coil 70 (as represented by the
 diamond sample D with dashed lines), the inductance of the tuned circuit
 changes (typically in the order of a few hundred nanohenries, depending on
 the amount of iron in the sample). This, in turn, changes the output of
 the sample tuned circuit 68. The outputs of the sample tuned circuit 68
 and the reference tuned circuit 66 feed reference point A and reference
 point B, respectively.
 Referring to FIG. 13B, a comparator 72 (left) compares the outputs of the
 two tuned circuits. The comparator 72 includes an operational amplifier
 configured as a differential amplifier U18. The output signal from the
 sample tuned circuit 68 (FIG. 13A) flows from reference point A through
 capacitor C26 and resistor R25 to one input of the differential amplifier
 U18. The output signal from the reference tuned circuit 66 (FIG. 13A)
 flows from reference point B through capacitor C27 and resistor R26 to the
 other input of the differential amplifier U18.
 Ideally, when the input signals of the differential amplifier are equal,
 the differential amplifier U18 should not generate an output signal.
 However, due to factors such as the variations that occur during the
 operational amplifier manufacturing process, differential amplifier U18
 typically generates a small output signal when a diamond sample D is not
 in the sample coil 70. To compensate for this signal, resistor R8 and
 potentiometer R7 provide an adjustment in the differential gain of the
 differential amplifier U18. Proper adjustment of potentiometer R7 can
 result in a null of 45 to 50 DB below the nominal signal output of the
 differential amplifier U18. The operation and construction of differential
 amplifier circuits are well known in the electronics art. Accordingly,
 these aspects of the circuit will not be discussed in detail.
 When a diamond sample D containing ferrous material is placed in the sample
 coil 70 (FIG. 13A), the differential amplifier U18 produces an output
 signal due to the small difference between the amplitude and phase shift
 of the signals at its inputs. Typically, the amplitude of the signal at
 the output of the differential amplifier is in the range of a few hundred
 microvolts. Depending on the ferrous content of the synthetic diamond,
 however, signals in the millivolt range are possible.
 The output of the differential amplifier U18 is amplified and filtered by
 the amplifier and filter circuit 76. The operational amplifier U19,
 resistors R31 and R32, and capacitors C28 and C29 comprise an active
 bandpass filter that filters out noise at frequencies above and below the
 frequency of the signal generated by the oscillator circuit 64 (FIG. 13A).
 In addition, U19 in conjunction with resistors R33 and R34 amplifies the
 signal generated by the differential amplifier U18. Typically, the
 amplifier provides a gain of approximately 1000:1. The amplified signal
 flows to reference point C (right).
 Referring now to FIG. 13C, an AC-to-DC converter 78 converts the
 alternating current ("AC") signal at reference point C (left) to a direct
 current ("DC") signal. Capacitor C32 and operational amplifier U20 provide
 coupling and DC isolation for the circuit. Diodes D6, D7, D8 and D9 form a
 bridge rectifier in the feedback path of the operational amplifier U20.
 This configuration eliminates the error caused by the diodes' voltage drop
 variation. Resistor R37 converts the current output of the diode bridge to
 a voltage. Capacitors C34 and C36 filter the signal output by the AC-to-DC
 converter.
 The output of the AC-to-DC converter is fed through reference points "+"
 and "-" (right) to a display device 74 (FIG. 13D). The display device 74
 is a single chip digital volt meter with a digital readout. The digital
 readout displays a number indicative of the probability that the diamond
 sample D (FIG. 13A) placed in the sample coil 70 is a synthetic diamond.
 This embodiment typically provides a more sensitive screening device than
 the first embodiment. First, the fixed frequency crystal controlled
 oscillator is not subject to as much frequency drift as the oscillator of
 the first embodiment. Second, the tuned circuit is more sensitive to
 changes in the effective coil parameters than many other comparable
 circuits. Third, the differential amplifier provides a very deep null and,
 as a result, can detect very small differences in the output signals of
 the tuned circuits. Moreover, the second embodiment can be constructed
 using relatively inexpensive components. Thus, this embodiment provides a
 relatively sensitive and inexpensive apparatus for screening diamond
 samples.
 Referring to FIG. 14, the third embodiment of the present invention screens
 a diamond sample by detecting a signal generated when the sample is
 rotated in a magnetic field. A motor assembly M, an air-gap magnetic head
 H and a magnetic head circuit 98 substantially provide the functionality
 of the magnetic field alteration detector 54 of FIG. 11. The remaining
 components of FIG. 14 substantially provide the functionality of the
 signal processor 62 and the output device 60 of FIG. 11.
 In this embodiment, the present invention is based, in part, on the
 realization that ferrous inclusions in a synthetic diamond are not evenly
 distributed within the diamond. As a result, the inclusions disturb a
 magnetic field when they are rotated within the field. Accordingly, the
 diamond sample D is positioned within a magnetic field generated by the
 magnetic head H, then rotated by the motor assembly M. When the diamond
 sample D is a synthetic diamond, the ferrous material in the diamond
 sample D disturbs the magnetic field thereby inducing a small alternating
 current signal in a coil 90 of the magnetic head H. This signal is
 amplified, filtered and processed to display the probability that the
 diamond sample D is synthetic.
 The motor assembly M is comprised of a motor 80, a chuck 82 and a shaft 84
 that is adapted to receive a diamond sample D. In general, to prevent the
 detected signal from being corrupted by ambient noise (e.g., 60 Hertz AC),
 the diamond must be rotated at a relatively high speed to produce a signal
 with a frequency that is higher than the ambient noise frequencies.
 Typically, the motor 80 operates at 25,000 to 30,000 revolutions per
 minute. The chuck 82 fastens the motor shaft 84 to the motor 80. The motor
 shaft 84 is a dielectric shaft, typically made of plastic, wood or other
 non-ferrous material. The diamond sample D is connected to the motor shaft
 84 using an adhesive 86 such as DOP wax or glue. Alternatively, any other
 suitable connecting method can be used. Typically, a small portable motor
 such as one sold by Dremel Corporation (commonly known as a "Dremel Tool")
 can be used for motor 80.
 The magnetic head H generates the magnetic field and detects a disturbance
 in the magnetic field caused by a rotating synthetic diamond. The magnetic
 field is generated by a DC current running through the coil 90 of the
 magnetic head H. This magnetic field is concentrated at an air-gap 92. In
 FIG. 14, the magnetic field is represented by magnetic field lines 94.
 The magnetic head H and the motor assembly M are mounted so the diamond
 sample D attached to the motor shaft 84 can be positioned near the air-gap
 92 of the magnetic head H. Typically, the distance between the diamond
 sample D under test and the magnetic head is in the range of a few
 thousandths of an inch.
 The magnetic head H can be constructed from relatively common components.
 For example, a read/write head from a relatively inexpensive tape recorder
 can generate acceptable signals. Tests have also indicated that a head
 with a relatively large air-gap may provide better results than a
 comparable head with a very small air-gap.
 A diamond sample D is tested by rotating the sample in the magnetic field
 generated by the magnetic head H. When the sample is a synthetic diamond,
 the deposits of ferrous material in the diamond disturb the magnetic field
 generated by the magnetic head H. This disturbance in the magnetic field,
 in turn, generates a signal in the coil 90 of the magnetic head H.
 Typically, the amplitude of the detected signal is in the order of one to
 ten microvolts. An amplifier and filter circuit 96 amplifies this signal
 to a suitable level for processing and filters the signal to remove noise.
 FIG. 15 depicts detailed circuitry for the magnetic head circuit 98 and the
 amplifier and filter circuit 96 of FIG. 14. A magnetic field bias circuit
 100 provides the DC current to the magnetic head coil 90 to generate the
 magnetic field. The values for resistors R39 and R40 are selected to
 provide the proper amount of current needed to generate an adequate
 magnetic field at the air-gap 92 (FIG. 14). Capacitor C37 provides a bias
 for the magnetic head current.
 The signal generated in the magnetic head coil 90 is coupled through
 capacitor C38 to the amplifier and filter 96. Operational amplifier U22
 and its associated resistors and capacitors filter out noise generated by
 the motor 80 and other sources. Capacitors C38 and C41 and resistors R41
 and R45 are selected for a bandpass around the desired signal
 frequency--typically in the 400 to 500 Hertz range. Alternatively, a notch
 filter may be used to filter the noise. The amplifier uses two operational
 amplifier circuits to provide the necessary amplification--typically on
 the order of 1,000:1. The components associated with operational amplifier
 U21 are identical to the components associated with operational amplifier
 U22. Resistors R43 and R44 and capacitor C40 provide gain and bias for the
 two operational amplifier stages.
 In FIG. 15, the signal processor 102 of FIG. 14 is replaced by an FFT
 signal processor 106. The FFT signal processor 106 provides additional
 noise reduction using "Fast Fourier Transform" signal processing. In
 practice, a null of greater than 100 DB can be obtained. Consequently, the
 use of the FFT signal processor 106 improves the sensitivity of the
 screening device. The operation and construction of FFT signal processors
 and associated noise reduction techniques are well-known and widely used
 in the signal processing art. Accordingly, these details will not be
 discussed further.
 After the amplified and filtered signal is processed by either the FFT
 signal processor 106 of FIG. 15 or the signal processor 102 of FIG. 14,
 the processed signal drives a display device 104. The display device 104
 generates a visual indication of the probability of whether the diamond
 sample is a synthetic diamond or a natural diamond.
 The third embodiment typically provides a more sensitive screening device
 than the first and second embodiments. First, because an oscillator is not
 used, the sensitivity of the circuit is not affected by oscillator
 frequency drift. Second, the circuit does not rely on a change in a
 circuit parameter (e.g., inductance). Thus, the circuit is not affected by
 the errors produced by the circuitry that detects changes in circuit
 parameters. Third, the FFT processing improves the sensitivity of the
 circuit when detecting very small signals in the presence of noise. Tests
 have demonstrated that this embodiment exhibits surprising sensitivity
 (typically in the order of a 100 DB signal-to-noise ratio) and, as a
 result, can distinguish between a natural diamond and a synthetic diamond
 that contains very little ferrous material. Thus, this embodiment provides
 a very effective yet relatively simple method for distinguish between a
 synthetic diamond and a natural diamond.
 From the above, it can be seen that the present invention provides a simple
 and accurate method of distinguishing between synthetic diamonds and
 natural diamonds.
 While certain specific embodiments of the invention are disclosed as
 typical, the invention is not limited to these particular forms, but
 rather is applicable broadly to all such variations as fall within the
 scope of the appended claims. To those skilled in the art to which the
 invention pertains many modifications and adaptations will occur. For
 example, while the oscillators and other circuits shown have been
 implemented with discrete components, they could readily be implemented on
 a programmable logic chip or chips as well. A variety of circuits could be
 used to construct the oscillator, comparator, amplifier, filter and output
 circuits described above. The change in the effective inductance of a coil
 could be detected using various filter circuits or signal response
 circuits. Various audio or visual devices could be used to indicate
 whether the diamond sample is synthetic or natural. A variety of
 mechanisms could be used to move the diamond sample in the magnetic field.
 Similarly, a variety of magnetic head devices could generate the magnetic
 field and/or detect changes in the magnetic field. Thus, the specific
 structures discussed in detail above are merely illustrative of a few
 specific embodiments of the invention.