CUBIC ZIRCONIA:AN UPDATEB y Kurt Nassau

Soon after i t was first marketed i n 1976,
colorless cubic zirconia became thedominant diamond imitation, withcurrent production of approximately 60
million carats per year. Although cubic
zirconia was discovered as a naturalmineral in 1937, crystals usable for
faceting were first produced ill 1969 andit was not until a practical sltull-melting
technique was developed i n the USSR i n
1972 that commercial production became
feasible. This article reviews the sl<ullmelting technique used to produce cubic
zirconio and examines the current statusof this diamond simulant with regard t oq ~ ~ a l i tproduction,y,
ond market. The
patent situation i s discussed, as well as
prospects lor new diamond imitations
and the recent surge of interest i n colored
cubic zirconia. Also inclzlded is an
Appendix summarizing the technical andgemological data for cubic zirconia andits distinguishing characteristics.

A BOUT THE AUTHOR
Dr. Nassau is a research scientist residing in
Bernardsville, NJ.
"'1981 Gemological Institute of America

Cubic Zirconia

ubic zirconia was discovered as a natural mineral in1937, when two German mineralogists, von StaclzC
elberg and Chudoba (1937), were examining a highly
metamict zircon given to them by B. W. Anderson. The
zircon contained some tiny crystals which they identified
by X-ray diffraction as the cubic form of zirconium oxide(or zirconia), a compound lznown as baddeleyite when in
the monoclinic form. So little did von Staclzelberg andChudoba think of this discovery that they did not evenassign a name to the new mineral. As a result, it is lznown
to this day by its scientific name, cubic zirconia, and theprefix synthetic, although proper, is not usually included.
This same material had already been used for many
years as a ceramic composition for high-temperature industrial and scientific purposes; because of an exceptionally high melting point, "stabilized zirconia" ceramics
can be used at temperatures up to 2540°C (4604°F)and arevery resistant to most chemical substances. Such stabilized zireonia typically consists of 96% ZrOs (zirconia)
and 4% CaO (lime), although MgO (magnesia) or Y203
(yttria)also can be used in place of the CaO. Further discussion of the early work on this material is given in
Nassau (1977).
Whenever the powder or ceramic form of a substancehas interesting properties, scientists usually attempt togrow large crystals; this permits a more precise study ofa broader range of properties and often leads to additional
uses. In the case of cubic zirconia, crystal growth proved
most difficult in view of the material's exceptionally highmelting point. Pure ZrOf melts at about 2750°C (4982"F),
but at a somewhat lower temperature when stabilizers
are added. Flux growth was tried (Nassau, 1977, 1980a),but it produced either monoclinic baddeleyite or a tetragonal form. On heating, the former is transformed frommonoclinic to tetragonal at about 1100°C (20!2"F), andfrom tetragonal to cubic at about 2000°C (3632°F).Un-

GEMS & GEMOLOGY

Spring 1981

9

Figure I . A large slzull-melting apparatus being operated b y George Braml~all.Courtesy o f
the Ceres Corp., Walthnm, M A .

fortunately, these changes reverse on cooling andthe starting material forms once again. The addition of stabilizers such as CaO, Mg.0, or Y,O:,
prevents these transformations on cooling: cubic
zirconia then remains the stable form down toroom temperature. Details of the different formsof zircoilia are given in the Appendix.
Crystals of cubic zirconia in a usable size were
finally reported in 1969 by Roulin, Vitter, andDCportes of France. They used a difficult form

10

Cubic Zirconia

of melting the material without a crucible to obtain crystals more than 15 nlm (0.6 inch) long, ofcomposition 87.5% Zr02, 12.5% Y203. Only a few
properties of the crystals appear to have beenmeasured at that time, however, and the potential
remained unrecognized. A somewhat modified
and greatly enlarged form of this apparatus wasused by Alelzsandrov, Osilzo, Prolzliorov, and Tatarintsev at the Lebedev Physical Institute of the
USSR Academy of Sciences in Moscow (1973).

GEMS & GEMOLOGYTheir technique, called "slzull melting," is described in detail in the next section. Crystals one
inch (2.5 cm) and larger in diameter can be grownwith a modern embodiment of this arrangement(fig. 1) without excessive difficulty. The impetus
for this work had been the hope-unfulfilled-offinding improved materials for laser or opto-electronic uses, but measurements of the crystals
showed a material that was hard, colorless, andwith optical properties that approximate the brilliance and fire of diamond better than any previous diamond imitation.
With its introduction into the gem trade latein 1976, cubic zirconia as a diamond imitation
immediately created a huge demand as well as
considerable anxiety. As with the introductionof every diamond imitation before it (Nassau,
1980a), some jewelers who did not know how torecognize the new material were unable to protect themselves. Nevertheless, examination techniques have now been adapted to readily distinguish cubic zirconia from diamond, and new
instruments have appeared to assist the gemologist-jeweler in this task, as outlined in the
Appendix.
As with YAG (yttrium aluminum garnet),
which was overproduced and pealzed about 1972
(Nassau, 1980a),we are now seeing a rapid rise in
the production of cubic zirconia accompanied bya rapid fall in the price. This supply-and-demand
movement, together with the recent introductionof colored forms of cubic zirconia and the curious
patent situation, is also discussed in the following
sections.
THE SKULL-MELTING TECHNIQUE
FOR GROWING CUBIC ZIRCONIA
The technique of crystal growth by solidificationfrom the melt in a container is one of the easiest
and least costly ways of obtaining large crystals.
When, however, a material has as high a melting
point and is as reactive as cubic zirconia, no container can be found to hold the melt, and a "selfcontained melt," "cold crucible," or "autocrucible" arrangement must be used. The techniquedescribed here was patented by Joseph F. Wenclzus
and coworlzers (19771, now one of the major manufacturers of cubic zirconia as the Ceres Corporation, Waltham, MA.
The apparatus shown in figures 2 and 3 consists of a cup-like arrangement made up of a circle
of copper fingers, water-cooled by complicated in-

Cubic Zirconia

Figure 2. Schematic of one form of skull-melting
apparatus; only some of the fingers
are shown.

Figure 3. The split halves of a small slzullmelting apparatus.

GEMS & GEMOLOGY

Spring 198111

ternal plumbing. There are small gaps between
the fingers. Surrounding this apparatus is a watercooled copper coil activated by a radio-frequency
generator, typically operated at a frequency of 4
MHz and at power levels as high as 100 1zW.
The small gaps between the metal fingers enable the radio-frequency energy to pass into thecup, which is initially filled with zirconia powder
mixed with the required stabilizer. The powder
is an electrical insulator at room temperature; itwill not begin to heat until after the addition ofsome pieces of zirconium metal, which are thenrapidly heated by the radio-frequency field. As thezirconia adjacent to the zirconium metal heats
up, it also begins to conduct electricity and melts
in a matter of minutes. The zirconium metal
reacts with the oxygen in the air to produce more
zirconia.
All the zirconia soon melts, except for a relatively thin sintered shell or slzin that remains
solid because it is cooled and prevented frommelting by contact with the water-cooled fingers.
In this way the molten zirconia is contained
within a jacket of its own powder, which also prevents contamination from the metal. More zirconia, again containing the required stabilizer, isadded to the molten material to fill the skull tothe desired level. A porous crust forms over the
top of the melt, as shown in section A of figure
4, and helps to reduce heat loss. The contents are
kept molten for some hours to ensure uniformity,
as well as to permit the vaporization of some impurities. The cup is then lowered very slowly outof the heating coil, typically over a 12-hour period. Useful crystal growth begins at the bottomof the melt. As can be seen in section B of figure
4, crystals nucleate and form parallel columns
near the bottom and grow upwards. They continue to grow until the whole melt has solidified,
as in section C, whereupon the power is shut off.
The growth process under way is shown in figure 1, and half of a "skull"-thesolidified crystals with the surrounding crust-is shown in figure 5. A "dopant," a purposely added impurity,
had been used to produce yellow cubic zirconia
in this growth run. Light tapping will separate thecrystal columns, which
as in figure 6. Annealing, typically at 1400°C (2552OFL in air or inoxygen may be used at times to remove residualstrain to remove any discoloration resulting froma slight oxygen loss, or to adjust the valence statesof impurities.

12

Cubic Zirconia

HIGH- FREOUENCYWATER- COOLED, ,"7

COOLING

1

COOLINGWATER
OUTLET

LOWER1NOMECHANISM

Figure 4, The process of solidification during
skull-melting crystal growth. A = formation o/a porozls crust over the top the melt, B =early growth of parallel columns of crystal
nucleate, and Ci = final stage, with the entire
melt solidified h y the crystal growth.

GEMS & GEMOLOGY

Spring 1981

Figure 5. Half of a large skull of yellow cubic
zirconia cryslals grown using the techniquedescribed in figure 4. Courtesy of the Ceres
Corp., Waltham, M A .

At first, a slzull 3 inches (7.5 cm) in diameter
was considered the state of the art for cubic zirconia crystals, but today slzulls 12 inches (30 cm)
in diameter and weighing 150 lbs. (68 kg), such as
the one shown in part in figure 5, are produced.
These yield about 100 lbs. (45.5 lzg or 227,000 carats) of usable rough. Individual crystals are wellover one inch (2.5 cm) wide and several inches
long; stones several hundred carats in weight havebeen faceted. The economy of large-scale production is significant, although the rather high costof the high-purity starting material is not muchaffected by such a scale-up.

:igure 6. Columnar crystals of cubic zirconia
me inch (2.5 cm) across.

Cubic Zirconia

THE CURRENT STATUS OF
COLORLESS CUBIC ZIRCONIA
Starting from a small amount in 1976, within justfour years production has grown to an estimated
13 tons (12,000 lzg or 60 million carats] of cubic
zirconia per year at the end of 1980. Corresponding to the figures for rough material, the annual
production of faceted material is estimated atabout 3 tons (2700 lzg or 13.5 million carats) per
year. During this same time period, the wholesale
price per faceted carat has fallen from an initial
US $40 or more to US $4 or less, varying of coursewith the quality of the material and the faceting,as well as with size, shape, and quantity.
It is interesting to compare the production
data for cubic zirconia with the data for synthetics previously used as diamond imitations, as
shown in table 1. Annual production of cubic zirconia has already well surpassed the YAG peakof 1972 which, it must be remembered, represented overproduction, was more than the market
could absorb, and resulted in the discontinuationof production by a number of manufacturers. A
reasonable guess would be that the equivalentstage for cubic zirconia is close at hand. A factor
that may prevent this from happening is the apparent opening up in recent months of the European market, where previous diamond imitations
never achieved significant popularity.
Many jewelry retailers at first showed a strong
resistance to handling cubic zirconia, but experience has now demonstrated that the simulant
has had no significant effect on diamond sales,
occupying instead a separate niche in the jeweler's range of goods. The available data for 1977
permitted an estimate of the retail value of all
diamond imitations sold in the United States ofabout $20 million, compared to about $2000 million for diamonds (Nassau, 1980a). Although by. now the sales figure for diamond imitations hasgrown significantly, so has the figure for diamonds, and the 1% ratio of diamond-imitation
sales to diamond sales has probably increased
only a little.
Major manufacturers of cubic zirconia in the
U.S. include the Ceres Corporation of Waltham,MA (marketing through MSB Industries of New
York City); ICT Corporation of Shelby, MI; Singh
Industries of Randolph, NJ; the LambdalAirtron
Division of Litton Systems of Morris Plains, NJ;
and Commercial Crystal Laboratories of South
Amboy, NJ. Outside the U.S. there is production

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Spring 1981

13

TABLE 1. The historical sequence of synthetics used as diamond imitations.

Synthetic
Sapphire
SpinelRutileStrontium titanate
YAG
GGG
Cubic zirconia

Year ofinitialuse

Peak annual
production of rough(in carats)

After 1905
About 1920
1948
1955
1968
1975
1976

?
?
750,000
1,500,000
40,000,000"
Small
60,000,000b

Year of peak
?
?
1955
1968
1972"
1976
1980-1 981

aPrernature peak due to overproduction.
bMay still be increasing.

in Moscow, USSR (marketed in the U.S. through
Clayhill Resources of New York City); at V.
Djkvahirdjian S.A. of Monthey, Switzerland [atfirst erroneously reported as using flux-growth[Nassau, 19761); as well as recent production in
Taiwan and undoubtedly elsewhere. The roughmaterial is faceted all over the world, but until
recently almost all has been used in the U.S. Despite the great improvement of cubic zirconia
over YAG (in brilliance) and over strontium titanate (in hardness), both of these other simulants
continue to be manufactured and sold, albeit on
a considerably smaller scale.
Claims are sometimes made that one manufacturer's cubic zirconia is better than another's,for example, in respect to turning dark in normal
use. Although some bad batches were undoubtedly produced in the early days, currently there
is no significant difference in the behavior of thehigh-grade material produced by the various manufacturers. Similarly, inclusions are virtually absent in top-grade material.
Names used for marketing cubic zirconia
include:CZ
Diamonique I11
CereneDiamonite or
Cubic Z
Diamondite
Cubic Zirconia
Djevalite
Cubic Zirconia I1
FianiteCubic Zirconium
Phianite or
Cubic Zirconium
Phyanite
Oxide or Dioxide
ShelbyDiamon-Z
Singh Kohinoor
Diamond-QU
Zirconia
Diamonair I1
Zirconium
Diamonesque
Zirconium Yttrium
Diconia
Oxide

14

Cubic Zirconia

Some of these names are registered trademarks, others are not. It is important to note that,by themselves, zirconium and cubic zirconium
are misnomers, since they would refer to zirconium metal! Also, some sellers are using the ending "-Z"with other terms, possibly to imply
cubic zirconia when this may not, in fact, be thematerial at hand.
The stabilizer used in cubic zirconia is usually
Y,03, although CaO has also been used. The phase
diagrams indicate that up to 65 weight percent
YzO3 or up to 14 weight percent CaO could be
used in cubic zirconia. Too much stabilizer
results in a softer and less brilliant product, however, and amounts much smaller than those indicated in the phase diagrams are used for thecommercial product, as described in the Appendix.
Now that large crystals of cubic zirconia arereadily available at a relatively low cost, scientists are once again studying the material for possible technological uses, thus coming full circle
to the original purpose for growing the crystals!
Cubic hafnia-hafniumoxide (HfO,) stabilized with Y,03-has also been described in the
cubic zirconia patents. This has about the samerefractive index as cubic zirconia, but with aneven higher specific gravity. Since hafnia is much
more expensive than zirconia, cubic hafnia is not
likely to be used as a diamond imitation.
THE CUBIC ZIRCOMA PATENTS
In December 1972, the USSR group of V. I. Alelzsandrov and coworkers applied for patents in the
U.S. and other countries, the relevant claim being
cubic zirconia stabilized with 10% to 30% yttrium oxide. British patent 1,373,888 was published on November 13, 1974, and German patent

GEMS & GEMOLOGY

Spring 1981

2,261,85 1 appeared on July 11, 1974. The U.S. patent application was abandoned and then refiledin March 1977, was again abandoned and then refiled in January 1978; it issued on May 8, 1979,
as U.S. patent 4,153,469.
Meanwhile, additional patents had been applied for in January 1975, leading to British patent
1,491,362, published on November 9, 1975, andGerman patent 2,501,800, published on July 22,
1976. The relevant claim in these patents was cubic zirconia stabilized with 5% to 10% yttriumoxide. Such a claim may also have been involvedin one of the U.S. refilings; if so, it was apparently
not allowed. It should be noted that the items
being patented are the chemical composition of
cubic zirconia crystals, not the growth technique.
After the issuance of the U.S. patent in 1979,
some retailers hesitated to purchase domestically
produced material. At least one U.S. producer (theCeres Corp.) guaranteed its customers that it
would defend them in any suits that might arise
from this patent. A suit based on this patent has,in fact, been filed; as of this writing it is still undecided. It should be noted that most cubic zirconia produced in the U.S. is believed to be madewith somewhat less than the 10% minimum
claimed in the patent. Even if this argument were
rejected, there is the fact that the patent contains
no mention of the cubic zirconia stabilized with
12.5% Y , 0 3 reported in 1969 (Roulin e t al.) andshown in large crystal form in a French film madeat that time; this would seem to imply that a
court test of the validity of the patent might wellresult in an invalidation.
COLORED CUBIC ZIRCONIA
Aleksandrov and coworlzers gave details on the
manufacture of colored cubic zirconia in their1972 patent application. Until recently, however,most manufacturers have concentrated on lzeeping color out of their crystals, since even trace
amounts of impurities such as iron can produce
a yellow hue, undesirable in a diamond imitation.
Some experimentation persisted, though, particularly to find good ruby red and emerald green
colors, neither of which seems to have beenachieved thus far. Along the way, significant
amounts of yellow and other colored material accumulated. Some was faceted by amateurs for the
fun of it and was found to be very attractive.
A major breakthrough in the market for colored cubic zirconia was apparent at the 1979 Tuc-

Cubic Zirconiason (Arizona) Gem and Mineral Show, where allavailable colored cubic zirconia rough was quiclzlybought up by amateur as well as professional cutters. The attractiveness of the colors lies in theircoupling with a high refractive index and dispersion, and can be seen to some extent in figure 7.
The optical constants of most colored gemstones-ruby,sapphire, emerald, amethyst, citrine, topaz, etc.-have relatively low values, andtheir attractiveness resides predominantly in thecolor. With colored cubic zirconia, however, there
is considerable brilliance, or "life," in addition to
the color; this liveliness can be transmitted only
poorly in even the best photograph.
A listing of the colors produced by specific dopants (purposely added impurities) is given intable 2, and the dopant compounds used to produce desired colors appear in table 3. The natureof the color can change with the concentration of
the dopant as well as with its oxidation state, andcombinations of dopants are used to obtain desired shades. Just some of the colors produced by
one manufacturer are shown in figures 7 and 8.
The manner in which allochromatic transition
metal impurities produce colors is described elsewhere (Nassau, 1974-75, 1980a, and 1980b).
The most frequently used colors at present are(1)the amethyst to lavender to lilac range, (2)theyellow (canary)to orange to reddish-brown range,and (3)pinlz. Colored cubic zirconia produces faceted stones that are much inore "lively" than theanalogous amethyst or purple sapphire for the
first color group and citrine, padparadsha sapphire, imperial topaz, or garnet for the second.
Prices for colored rough are somewhat higher thanfor the colorless. At present, these colored stones
seem to be used primarily for fad-type mediumpriced fashion jewelry. The quantity of colored
cubic zirconia produced is still quite small but isincreasing rapidly and may soon represent a significant portion of the marlzet. Attractive intense
greens, blues, and reds have not been produced
thus far.
Were i t not for the relative rarity of coloreddiamonds and the fact that most people are notfamiliar with them, these colored cubic zirconias
would provide excellent imitations for naturally
occurring canary, pinlz, blue, and green diamond.
Jewelers should indeed be aware that, with thecorrect shade and intensity of color, these cubic
zirconias could easily be mistaken for coloreddiamonds.

GEMS & GEMOLOGY

Spring 1981

15

Figure 7. Faceted cubic
zircoz~ias;the largest is 9mm across. CourLesy of the
Ceres Corp., Waltham, M A ,and MSB Industries, New
York, NY.
TABLE 2. Colors produced by specific dopants added
to cubic zirconia."

TABLE 3. Dopanls used in cubic zirconia to givedesired colors.

DopantColor range

CeriumChromium
CobaltCopperErbiumEuropium
Holmiuml ron
Manganese
Neodymium
NickelPraseodymium
Thulium
Titanium
Vanadium

SymbolColor
Yellow-orange-red
Olive
Lilac
Yellow
Pink
Pink
Pink
Yellow
Brown-violet
Lilac
Yellow-brown
Amber
Green
Yellow-brown
Green

aTypically at the few tenths of a percent level; the color
achieved may depend on the oxidation state of thedopant.

16

Cubic Zirconia

Dopants used

Yellow-orange-red
Yellow-amber-brown
Pink
Green-olive
Lilac-violet

CeO,, Ce203
CuO, Fe203,NiO, Pr203,TiO,
Er203,Eu203,Ho203
Cr203,Tm203,V203
C0203, MnOa, Nd203

PROSPECTS FOR THE FUTURE
In chapter 1 and the epilogue of my book (Nassau,
1980a), there is an extensive discussion of thesources of new synthetics as well as of the way
in which these sources have been changing over
the years. Today, only the most difficult of thesyntheses have not yet been achieved and, as crystal growth techniques have become ever moresophisticated, the search for new crystals has become more costly and time consuming.

GEMS & GEMOLOGY

Spring 1981

Figure 8. Colored cubic
zirconia rough; the largest
piece is 2.5 inches (6.5 cm)
long. Courtesy of the Ceres
Corp., Waltham, MA.

It is, therefore, hardly a coincidence that themost recent diamond imitations-YAG, GGG(gadolinium gallium garnet), and cubic zirconiaall originated as spin-offs from technological research. The use of YAG as a laser material is welllznown, GGG was developed as a substrate formagnetic bubble memories, and cubic zirconia
was studied as a possible laser or opto-electronicmaterial. Is it then lilzely that this source willproduce another material that might provide a
diamond imitation that is superior to cubic zirconia?
I believe the answer to this question is yes, but
a qualified yes. Although such a material willundoubtedly appear sooner or later, it must be realized that technological research for new materials has slowed significantly. In part, this isbecause present needs have been largely filled andthe areas left to be explored are becoming smaller;in part, it is because existing materials are being
studied intensively so as to utilize their poten-

Cubic Zirconia

tials more fully. So it may not be all that soonthat a potential successor to cubic zirconia will
arrive.
Further qualifications arise from two factors.
First, the optical constants of cubic zirconia aresufficiently close to diamond in a material of adequate hardness and wearability that a large improvement cannot be expected. This was hardly
true of any of the previous diamond imitations.
Second, the cost involved in developing and marketing a new synthetic or imitation is never smalland, given the existence of a highly satisfactorymaterial in the marketplace, may not be justifiable in terms of the potential returns. (One may
note the difficulty synthetic flux-grown rubies
have had in achieving market penetration in competition with the Verneuil-grown product; they
recently came close to success by posing as natural rubies with forged GIA certificates!) All inall, it would seem that cubic zirconia is lilzely to
be with us for quite some time.

GEMS & GEMOLOGY

Spring 1981

17

APPENDIX: TECHNICAL ANDGEMOLOGICAL DATA
AND IDENTIFICATION

Inclusions. Small particles (Nassau, 1976, 1977, and1980a; Liddicoat and Koivula, 19781, gas bubbles (Nassau, 19761, and a curious striated appearance (Nassau,
1976, 1977, and 1980a) can sometinles be seen but areabsent in good-quality material.

Composition. Typically zirconia (zirconium [diloxide,
ZrO,] containing about 10 mol. percent yttria (yttriumoxide, Y,0,,] or 15 mol, percent lime (calcium oxide,
CaO). Zirconia can exist in several polymorphic structures, as listed in table 4.

Effect of Exposure to Ultraviolet and Other Irradiations. Variable fluorescence, may be completely absent; often turns brown, depending on cluality, nature,and length of irradiation. Good-quality material willagain fade to colorless when placed in sunlight for a few
hours and will not darken in sunlight, that is, "solarize."

Structure. Same as that of fluorite, CaF,, with Zr inplace of Ca and 0 in place of F. This structure, shownin figure 9, consists of a simple cubic arrangement ofoxygens with every other cube occupied by a Zr. Each
0 is surrounded by four Zr, each Zr by eight 0. When
ZrO, is stabilized with either Y203 or CaO, there arefewer than two oxygens per metal atom, and some oxygens are missing: one 0 for every two Y or one 0 for
every Ca, as is implied in the typical formulations
Zr0.9,,Y0.1001.93
and Zro.8,Ca0.1501.8j.
Sonle Zr and somedopants may have only seven or six 0 around them instead of the usual eight; this changes the crystal field
and, accordingly, the color (Nassau, 1974-75, 1980a,
1980b).
Crystal Symmetry. Cubic; class, 41m321m;space group,
Fm3m.
Optical Properties. Refractive index, 2.15 to 2.18; nobirefringence, singly refracting; dispersion, 0.058 to
0.066.
Mechanical Properties. Hardness, 8.0 to 8.5 (a slightanisotropy in hardness appears to be present; this ispossible even in the cubic system and also occurs indiamond); specific gravity, 5.6 to 6.0; no cleavage; conchoidal to uneven fracture; somewhat brittle; goodwearability.

I

A T EACH CORNER: 0

Figure 9. The crystal structure of cubic zirconia.

TABLE 4. The polymorphs of zirconium oxide.

Structure

Temperature
range (°C)a

Unit cellparameters (A)Specific
gravity

Monoclinic P2,Ic

Up to 1100

5.83

Tetragonal P4dnrnc
Hexagonal (?)
Cubicb Fm3m
Cubicc Fm3m
Cubicd Fm3m

1100 to 2000
About 1900
2300 to 2750
Up to melting
Up to mellinga = 5.15, b = 5.21
c = 5.31, P = 9g023'
a = 3.64, c = 5.27
a = 3.60, c = 5.90 (5.88)
a = 5.26
a = 5.12
a = 5.12 to 5.16

5.86
6.13
5.64
6.00
5.60 to 6.00

"The exact lemperatures depend on the impurilies present and the amounts thereof.
bThe unslabilized high-temperature form of pure ZrO,.
"The natural stabilized material of von Stackelberg and Chudoba (7937).
dThesynthetic stabilized material.

18

Cubic Zirconia

GEMS W GEMOLOGY

Spring 1981

I

Distinction from Diamond. Several articles in thisjournal have dealt with various aspects of this problem(Liddicoat and Koivula, 1978; Nassau, 1978-79a, 197879b, 198 1; Nassau ancl Schonhorn, 1977-78; Shaw,
1978). The distinction is obvious to the trained, awareeye. In a loose stone, the high specific gravity is readily
apparent. Flatness of faces and sharpness of edges are
not foolproof criteria, and girdles apparently showing~lilaturals"have been observed on cubic zircoilia (Liddicoat and Koivula, 1978; Nassau, 1978-79b). The
uniquely high thermal conductivity of dianlond provides an uilan~biguousidentification of diamond whena well-compensated instrument such as the Ceres Diamond Probe (Nassau, 1978-79a) is used, but soille careis required with a simpler instrument such as thepocket-sized Ceres Czecl<mate. A colnprehensive testof such instruments, including the GEM Diamond
Master is under way (Nassau, to be published). The
breath test is a zero-cost version of the thermal conductivity tcst but recluircs conlpiirison stones (Nassau,

1976-79b). Although there are some limitations, oneof the many reflectivity meters should give a clear distinction (Liddicoat and Koivula, 1978; Nassau, 197879b). Finally, the wetting contact angle (Nassau andSchonhorn, 1978-79) provides a distinction when the
GEM Diamond Pen or one of a number of other pensis used (see Shaw, 1978, for a discussioil of the GEM
Diamond Pen, and Nassau, 1978-79b, for a discussionof that and other pens), but surface coatings may haveto be removed. As always, the gemologist must guard
against the possibility of a doublet.

Size, Weight, and Shape. Proportions a i d cuttiilg techniques have beell discussed (Kerr, 1979). A graph for the
rapid conversion of size to weight and to the equivalentdiamond weight has also been given (Nassau, 1979 and1980a). It should be noted that cubic zirconia stones
may be sold by size, by weight, or by equivalent diamond weight (the last is not always so specified].

REFERENCES
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Cubic Zirconia

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