A NEW DIAMOND SIMULANT --

Synthetic Moissanite

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 Mohs 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 tablein 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.

Recent Update

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.

Synthetic Moissanite : 4C and Its Features

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.

 

Testers

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

1. Scope of application : for diamonds, from colorless to fancy light color (S & T color grade).

2. Principles, Structures and the How-to: Making use of the following properties: colorless and light colored diamonds pertain the penetrability for long wave ultraviolet rays, and synthetic moissanite has its absorbency. So when a bulb sends long wave ultraviolet rays into the diamond to be tested, if the diamond is either colorless or with light colors, the long wave ultraviolet rays would refract from inside the diamond, then, through the table, enter a thin fiber-optical tube-like receptor, which would send out a "di-di" sound and the green light turns on. This refraction does not work for any intermediate or dark colored diamonds or to synthetic moissanite -- because they could have absorbed the ultraviolet rays from inside other than refract them. So fiber-optical tube would not receive the signal, thus the tester remain soundless --- without any reaction. Notice, before using the tester, just sort out / clear away all other diamond simulants --- by using the thermo-detector --except genuine diamonds or synthetic moissanite .

3. Strong & weak points: Clearly effective on colorless or light-colored diamonds, but it is ineffective on medium and darker colored diamonds --- regardless of natural or treated color or synthetic colored diamonds. And this might give rise to certain problems in selling colored diamonds for jewelry store owners.

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

1. Scope of application: all diamonds.

2. Principles, Structures and the How-to. It is made on the basis of the difference between the hardness of diamonds --- 10 on Moh's Hardness Scale, and that of synthetic moissanite --- 9.25. Diamond grit are plated onto the pen-point (Fig. 4-13); while testing, press the pen-point with proper effort to cut/mark on the girdles of the diamond to be tested. Then to see the result with a loupe, for no mark would be left on diamonds, and there must be some white concave marks on any synthetic moissanites or on any other simulant diamonds.

3. Strong & weak points: It is used to differentiate diamonds from any and all simulants ---including synthetic silicon carbides. Yet it belongs to destructive test, and its uses are only limited to experienced appraisers and dealers. It is not applicable for the majority of dealers and customers as well.

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

1. Scope of application: diamonds, simulants, gems.

2. Principles, Structures and the How-to : Using the theory that various substances do have different refractive indexes as well as reflective indexes. Thus, because the reflective index varies, as the table of the diamond to be tested is shed light on, the reflection with its respective scope will help tell us in general the right category of the gem: diamond's index is 17.2%; synthetic moissanite is about 21.0% (test is done from the table). Also notice, before using the tester, it is necessary to sort out all other simulants --- by thermo-detector, not including diamonds and synthetic moissanite. Otherwise, cases might be quite confusing: strontium titanate has a similar reflection scope as that of diamonds, so does synthetic rutile, which has a similar reflection scope as that of synthetic moissanite.

3. Strong & weak points : It is good to differentiate diamonds, simulants, and gems --- those with high refractive index. Yet the tester of the first generation has its weak points in designing. For instance, if the well inlaid diamond has a higher claw than its table, it is impossible to receive light properly. If the table is not big enough to receive reflective light, the testing has no positive effect. Now the manufacturers are trying to get it improved by producing the 2nd generation of the tester.

Presidium replace the reflection percentage into digit. Synthetic moissanite is 100 and above. Diamond is At 89 to 96. All other gem stones should have normal polished girdle before testing. Few more caution points are that the table of the stone should be well cleaned; and the stone or mounted jewelry should be covered in dark or near dark environment during the test.

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

The diamond is not have electro-conductivity, except for type IIb which is the blue diamond. The blue diamond contains baron which makes the stone becomes semi-electro-conductive. Synthetic moissanite is electro-conductive. Before using this tester, thermo-detector should be used to sort out all the other simulants, except for genuine diamonds or synthetic moissanite.

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

This kind of tester combines thermo-detector and reflectivity tester. The use is similar to Presidium.


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.

 

Conclusion

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.

 


Table 1

 

New Substitute for Diamond Synthetic Moissanite

 

Diamond

Cubic Zirconia

Synthetic Moissanite

Nature of Material

C

ZrO2

SiC (Al, Fe, Ca, Mg)

Crystal System

Cubic

Cubic

Hexagonal (Uniaxial positive)

Color

Varied

Varied

Grayish Blue / Grayish Green, Yellow

Refractive Index

2.417 (SR)

2.09-2.18 (SR)

2.65-2.69 (DR)

Dispersion

0.044

0.060

0.104

Ultra-Violet Fluorescence

None/ Blue/ others

Yellow/Orange

None

Gravity Index

3.52

5.6-6.0

3.22

Hardness

10

8.25

9.25

Inclusions

All kinds

Bubble

Metal ball shaped, or multiple whit lines

Electro-Conductivity

Non-Conductive

Non-Conductive

Conductive

Dichroism

None

None

None

Thermo-Detection

With diamond reaction

Without diamond reaction

With diamond reaction

Double Refractive Image

None

None

Yes