Apparatus for testing diamonds

Apparatus for testing diamonds for genuineness including a housing, a probe tip mounted in the housing and formed of a conductive material, a voltage stabilizer located in the housing remote from the probe and coupled to the probe tip by conductive material, the voltage stabilizer being operative to provide an output voltage which varies as a function of the temperature of the conductive probe tip, and indicating apparatus operative to provide a sensible output indication of genuineness of a diamond in response to the output voltage from the voltage stabilizer.

FIELD OF THE INVENTION 
The present invention relates to testing apparatus and more particularly to 
testing apparatus for crystalline articles, such as diamonds. 
BACKGROUND OF THE INVENTION 
Various types of testers for crystalline substances are known in the prior 
art and in the patent literature. These include apparatus for determining 
the genuineness of diamonds employing conductivity measurements. Such 
apparatus is exemplified in U.S. Pat. Nos. 4,255,962; 4,364,677; and 
4,344,315 all of which employ temperature sensitive elements, such as 
thermistors, located at the tip of a probe, which tip is firmly contacted 
with a diamond to be tested. The thermistors measure the thermal response 
of the probe tip. Devices of the type described in the aforesaid U.S. 
Patents are relatively expensive, sensitive and complex electronically. 
SUMMARY OF THE INVENTION 
The present invention seeks to provide an improved diamond testing device 
which is significantly less complex and expensive than the prior art 
devices described hereinabove. 
There is thus provided in accordance with an embodiment of the present 
invention apparatus for testing diamonds for genuineness comprising a 
housing, a probe tip mounted in the housing and formed of a conductive 
material, a voltage stabilizer located in the housing remote from the 
probe and coupled to the probe tip by conductive material, the voltage 
stabilizer being operative to provide an output voltage which varies as a 
function of the temperature of the conductive probe tip, and indicating 
apparatus operative to provide a sensible output indication of genuineness 
of a diamond in response to the output voltage from the voltage 
stabilizer. 
Further in accordance with an embodiment of the present invention there is 
also included in the diamond testing apparatus, a device for sensing the 
pressure of the probe tip on the substance being tested and apparatus for 
calibrating the sensed conductivity of the substance being tested in 
accordance with the sensed pressure. 
Additionally in accordance with a preferred embodiment of the present 
invention, the voltage stabilizer comprises a precision temperature 
stabilized zener diode. 
Further in accordance with a preferred embodiment of the invention, the 
precision temperature stabilized zener diode LM 399A manufactured by 
National Semiconductor.

DETAILED DESCRIPTION OF THE INVENTION 
Reference is now made to FIG. 1 which illustrates testing apparatus 
suitable for conductivity testing of the genuineness of diamonds and 
comprising a housing 10 defining a body portion 12, a grip portion 14 and 
a tip portion 16. Disposed in body portion 12 of the housing 10 is test 
circuitry 18, which will be described hereinafter in greater detail with 
reference to FIG. 2. 
Disposed within the grip portion 14 of the housing 10 and electrically 
coupled to circuitry 18 is a thermal sensor 20, which according to a 
preferred embodiment of the invention comprises a precision 
temperature-stabilized monolithic zener diode, such as an LM399. The LM399 
is preferred because it operates at an elevated temperature and provides a 
highly precise voltage change in response to temperature losses produced 
by contact with a highly conductive element such as a genuine diamond. 
Thermally coupled to the thermal sensor 20, as by a good thermal conductor 
22, is a probe element 24, typically formed of copper or any other 
suitable conductor, which includes an elongate portion 26 which extends 
through the tip portion 16 of the housing 10 and defines at its extreme 
end, a contact tip 28 for engagement with a substance to be tested. 
Reference is now made to FIG. 2 which is a circuit diagram of a preferred 
embodiment of testing circuit useful in the apparatus of FIG. 1. The 
circuit is based on four operational amplifiers 30, 32, 34, and 36 which 
are conveniently embodied in a single integrated circuit, such as a 324 of 
National Semiconductor. Briefly stated, the testing circuit is arranged to 
sense the conductivity of the subject substance being tested. When the 
contact tip 28 of the probe element 24 contacts a good heat conductor such 
as a diamond, the temperature of the thermal sensor 20 drops and the 
output voltage of the thermal sensor 20 changes. Because the thermal 
sensor 20 is temperature-stabilized by action of a heater contained within 
the thermal sensor 20, the change in the temperature of the thermal sensor 
20 caused by the contact of the probe element 24 with the diamond turns on 
a heater contained within the thermal sensor 20. Eventually, the heater 
increases the temperature of the thermal sensor 20, restoring the voltage 
output of the thermal sensor 20 to its initial level. However, in the 
interim, the contact of the contact lip 28 with a heat conductor causes a 
voltage transient in the output of the thermal sensor 20. A good heat 
conductor such as a diamond produces a relatively large voltage transient. 
The magnitude of this transient is compared by the circuitry 18 with a 
reference voltage, and if the voltage transient is sufficiently great, 
indicating that the object tested has the thermal conductivity of a 
genuine diamond, the circuitry 18 gives a sensible indication of this 
fact. In order to take into account the size of the mass of the substance 
being tested, there is provided a switch 38 for adding additional 
resistance to the circuit when the switch is closed, in order to 
accurately test relatively large mass substances, such as large diamonds 
of the order of 0.15 carat or more. 
An LED 40 is provided at the output of operational amplifier 30 and is 
operative to illuminate when the temperature at the probe element 26 is at 
a predetermined desired initial temperature suitable for beginning 
testing. 
A piezoelectric crystal sound transducer 42, which is commercially 
available from Phillips of the Netherlands, is provided along a feedback 
loop between the output of operational amplifier 34 and the negative input 
thereto and is operative to provide an audio output when a genuine diamond 
is tested. An LED 44 is also coupled to the negative input of operational 
amplifier 34 and is operative to illuminate in response to the sensing of 
a genuine diamond by the testing circuit. 
Both the constructional principles of the testing circuit and the criteria 
for determining what conductivity is indicative of a genuine diamond are 
well known in the prior art, inter alia from the prior art patents 
described hereinabove. Operational amplifier 30 is operative to illuminate 
LED 40 when the thermal sensor 20 reaches a predetermined temperature 
which is suitable for testing. Operational amplifier 32 functions as a 
comparator to determine the threshold at which conductivity of a genuine 
diamond activates the device. Operational amplifier 36 functions in 
response to the output of the operational amplifier 32 to illuminate LED 
44 in the sensed presence of a genuine diamond. Operational amplifier 34 
is operative to cause activation of the transducer 42 in response to the 
output of operational amplifier 36. 
Switch 38 is operative to vary the resistance of the reference of 
operational amplifier 32 by selectively switching additional resistance 
into and out of the circuit. 
Calibration of the testing circuitry is achieved empirically by adjusting 
the potentiometer at the output 10 to operational amplifier 32. 
Reference is now made to FIG. 3 which illustrates the probe construction of 
an alternative embodiment of the invention. Here a tip element 50 is 
spring mounted for axial movement with respect to the tip portion 52 of a 
housing 54. A strain gauge 56 is connected along tip element 50 for 
sensing the amount of force exerted thereon. The output indication of the 
strain gauge or of any other suitable force measuring means is supplied to 
the testing circuitry and is used to calibrate the testing circuitry 
according to the applied force between the tip and the substance being 
tested, since the measured conductivity is a function of this applied 
force. 
In the embodiment of FIG. 2, the strain gage 56 of FIG. 3 may be connected 
in parallel with switch 38 as shown in FIG. 2. Strain gage 56 is operative 
to vary the reference resistance at input 10 of operational amplifier 32 
as a function of the pressure exerted on the tip. In this manner, 
compensation is provided for differences in the pressure applied by 
various users, since the measured conductivity varies somewhat as a 
function of the pressure of the tip applied to the substance being tested. 
It will be appreciated by persons skilled in the art that the present 
invention is not limited by what has been particularly shown and described 
hereinabove. Rather the scope of the present invention is defined only by 
the claims which follow: