By Joe C. C. Yuan -- G.G., D.G.A.

According to the recent report made by several jewelry magazines, an entirely new diamond simulant named silicon carbide, synthetic moissanite has been mass produced by the C3, Inc. in North Carolina in the USA, and it can be used to substitute CZ as a new diamond simulant. The author was trying to probe into this matter and did not get the sample until November, 1997. Soon various tests in the labs were made at China University of Geosciences (Wuhan City) of China and at the Research Institute of Geophysics of Academia Sinica in Taiwan. For your references, the author also made a comparison and enlisted varied properties of CZ vs. genuine diamond.

Name: Synthetic - Moissanite II

1. Composition: SiC contains a little trace elements impurities, like aluminum, iron, calcium, magnesium. Formerly, both qualitative and quantitative analyses were conducted by using electron probe micro-analyzer; the obtained result ( Si about 65%, C about 35%) was not always ideal due to the lack of advanced instruments and better samples for the data of other trace elements. Therefore, the error value was higher than expected. The information about the aforesaid elements could only be obtained from some mineralogical data.
2. Crystal system: It is known as hexagonal system. Under the polarization microscope, it presents a cross figure on the glass club, and is uniaxial. Put across the middle of the cross with a quarts wedge, the cross line is split outwardly. This is positive. This sample however, is of -type silicon carbide. It would be more difficult for the appraisal, if the sample should be -type, which appears to be a cubic crystal in the mineralogical data. Besides, the data also shows that the uniaxial positive of such substance belongs to Moissanite II.
3. Color: Now the synthetic silicon carbide, with its size, transparency of single crystals to be used as a substitution of diamonds, has a color variety of grayish blue, grayish green and grayish yellow. Because of the difficulties in manufacturing technology, colorless crystal are not available to the public.
4. Refractive index: The Refraction Tester in common use for diamonds can only reach 1.80, and the professional microrefraction tester at Geosciences University of China (Wuhan) also can only reach 2.45. During the experiment, the pointer did vigorously bump toward the scale limit and could not go beyond the extreme of 2.45 on the tester. What could be done was to have to use a large instrument - microreflect meter and to get the reflective index and to calculate the refractive index. Only after repeated tests, the more reliable range was found between 2.65 to 2.69, which proven to be strong birefringence. When the polished gemstone was placed face down on a sheet of plain paper with a thin black line, It was just like the real diamond, which made the black line not easily visible from bottom.
5. Dispersion: As the microreflector failed to give an accurate data in the testing with red and blue filter, we may have the data from G&G of GIA in the winter of 1997 for reference: 0.104.
6. Ultraviolet fluorescence: There was no fluorescent stimulation at all under the irradiation of long and short wave ultraviolet rays.
7. Specific gravity: Immersed in carbon tetrachloride under the temperature of 16 C – for gravity, then, after correction and conversion, the correct gravity was 3.22.
8. Hardness: Hard enough to cut or engrave on corundum, which is about 9.25 on Moh’s scale. So each facet could be well polished, made flat and smooth, and with sharp ribs.
9. Inclusions: With dark colored metal glitters, three were ball-like shaped and arranged a line. From these we know they were such made out of a deposition action. On the parallel optic axial direction, there were a lot of some white spots composed of thin lines.
10. Conductance: This material has electroconductivity.
11. Dichroism: It does not have dichroism, as was the result of observation with dichroscope.
12. Thermodetector action : It has the same action as genuine diamond when tested by thermodetector.
13. Double refractive image: Because this material is of hexagonal system with birefringence, even though we must use that facet with vertical optical axle to be the table—in cutting and polishing, so that when looking down from the table, there are not any double refractive images are visible if looking at the ribs near the culet through the kite facet or upper girdle facets.

In mid-September of 1998, the author made a trip expressly to visit the C3, Inc. in North Carolina, where he had significant discussions with leaders of the company and directors of different departments. The author made valuable suggestions on the angles and proportions in the cutting process for round, as well as moissanite in princess cut. In early October, he visited an important C3's processing factory in Qingdao, China, where he also made some suggestions on various cutting techniques.

Cutting: the designing of C3 vs. the Ideal Cutting calculated by the author.

1998	Crown Angle	Bottom Angle	Table
1Q & 2Q of 1998	280	400	63-70%
3Q of 1998	280	400	58-62%
4Q 1998 until mid-1999	300	400	54-57%
Ideal Proportion	320	40.750	54-56%

Girdle: Those processed in 1st and 2nd quarters were not so flat yet smooth; and those after the 3rd quarter 1998 were like Israeli polished girdle -- which appear to be cylinder-like with bright, transparent girdle (Fig. 4-1).

Weight: Sale is conducted and calculated in millimeter unit of diameters. Because of the cutting and polishing of crown, bottom angles that are usually rather shallow, plus the reason that its gravity is 10% lighter than that of diamond, the standard cutting of 1 carat diamond is of 6.5 mm in diameter, yet the synthetic moissanite is only over 0.80 carat.

Color: Up until today, it is still beyond the technological ability to produce colorless crystals. So on the market now are only light grayish green and light yellow. If judged by GIA standards, it is about H.I. -- P.Q. (Fig. 4-2).

Clarity: Its crystallization will make it a 2-3 inch diameter high cylindrical crystal , some of which can reach the standard of IF, while most of it contains white lines, which can reach VS -- SI. They are roughly vertical to the table, yet not strictly parallel to each other, of which the difference is by a few degrees (Fig. 4-3 & 4-4). Only a very little part contains dark colored, metal ball-shaped inclusions.

Double refractive images: Diamond is of cubic crystal system with single refractivity, while synthetic moissanite is of hexagonal with strong double refractivity. When being cut, simply place the optic axis vertically against the table. For synthetic moissanite, double refractive images of

ribs near the culets are visible --- if looking through the kite facets or upper girdle facets. As is seen here in Fig. 4-5, there is a lotus image under the table. Fig 4-6 shows : diamond presents no double refractive images; yet there are double refractive images in Fig 6-7, hence it is synthetic moissanite. Same as Fig. 4-8, when looking at the reflection of the girdle from the table at a tilted angle.

Ribs: The hardness of synthetic moissanite is 9.25°. With nice polishing, the ribs can also be sharp enough, like in Fig. 4-9. If compared with diamond (Fig. 4-10), it is not easy to tell their differences.

I. Tester Model 590 (Fig. 4-11) made by C3, Inc.:

II. Hardness Testing Pen (Fig. 4-12)

III. The Reflectivity Meter by Presidium (Fig. 4-14 & 4-15) :

IV. Electro-conductivity Tester - The Moissketeer 2000 (Fig. 4-16 & 4-17):

V. Thermo & Reflectivity Tester – ABC Diamond Pro (Fig. 4-18):

Here are also some more advance machines for the detection of moissanite:

DiamondView™ : The author also used DiamondView™ to check for synthetic moissanite. The image (Fig. 4-19) shows six rays of light caused by its hexagonal crystal system.

FTIR: Moissanite is identified on transmittance diagram with peaks at 1475 cm^-1, 2220 cm^-1, 2260 cm^-1, 2485 cm^-1, and 3720 cm^-1.

Raman Spectroscopy: Along the optic axis, sharp Raman peaks are located at 160 cm^-1, 766 cm^-1, 782 cm^-1, and 986 cm^-1. Weak peaks are observe at 95 cm^-1, 240 cm^-1, 260 cm^-1, 503 cm^-1, 1525 cm^-1, and 1720 cm^-1.

Synthetic moissanite has its superiority in its optical features and hardness, etc . If only the cutting and polishing are nicely done and with a correct proportional angle, the optical effect could be much better than that of genuine diamonds. Besides, for good appraisal work, dealers should have a good understanding of its specialties, and make a good observation of samples, so that his own diamond business can be well protected, or he could be able to go in for a special business for selling synthetic moissanite.