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SYNTHETIC MOISSANITE SPECTRA

And Selected Commentary

SAS2000: Developed, Produced And Supported In The USA   Questions, Comments, Product Support
Adamas Gemological Laboratory is proud to introduce to the industry the custom built SAS2000 Spectrophotometer Analysis System for diamond and gemstone evaluation and grading. The SAS2000 provides the most accurate colorimetry available today for diamond color grading, helps determine radiation treatments of diamond, provides better ability than the DeBeer's DiamondSure to detect probable  synthetic diamonds , and replaces the spectroscope for transparent gemstone evaluation.
NEW DATA
Silicon carbide is also being grown in  Russia , but the quality leaves a lot to be desired at this time. C3's patents preclude this material from being legally imported and sold in the United States.
Samples from C3 current distribution of synthetic moissanite were just tested on the SAS2000 which showed direct correlation with optical spectra and resistivity. Resistance (conductance) was measured on a Micronta Model 22-194 digital multimeter 9 VDC power source) and indicated on 14 of 16 samples electrical conductivity, with measured resistance of the samples in the 1 to 10 Meg ohm range. The polarity of the VoltOhmMeter had to be switched, as in one direction resistance was greater than 20meg ohm which was the limit of the resistance measurement of the ohmmeter. The other two samples showed resistance levels higher than 20Meg ohms, regardless of the lead polarity. Because Silicon Carbite aka Moissanite is a Semiconducter, this was not unexpected.
The same samples had been tested with a GIA conductometer (vintage 1970's) which was a 150VAC panel meter connected in series with the moissanite samples and driven by the115VAC output of an isolation transformer. Again the same 2 moissanite samples indicated no conductivity (0 Volts) while the other 14 samples registered 50-70 Volts.
When subjected to 1000V with a  BK Precision Model 300 Electrical Insulation Tester the samples indicated 1-5Meg Ohm.. OK then electrical breakdown voltage of the 2 high resistance moissanite samples was then interpreted to be between 115 and 1000 Volts.
The following curves show two grouping of moissanite spectra, the representative transmittancecurves with the dip at approximately 625nm were the conductive(C) samples and the two non conductive samples had a flat transmittance from 600 to 850nm..  Impurities in the sample correlating with resistivity and optical spectra.
 
 
PREVIOUS DATA

We have recently examined two synthetic moissanite specimens, and present the spectra below for your evaluation. These data were taken on two green samples, one a tabular crystal rough of older research production and one a faceted sample of recent production. Both exhibited a cutoff at about 425nm, which was consistent with a colorless sample examined last year in Tucson, but whose sample spectra was not kept. We will correct this error next week, if samples become available. The colorless moissanite spectra, as we remember, looked like a long wave bandpass filter (step function) with the same approximate 425 nm cutoff as the green spectra.

Detection of colorless moissanite should be no problem for those with rudimentary gemological skills, because of its very high double refraction, greater than that of Peridot. If one examines the pavilion facet junctions of an unknown colorless specimen using darkfield illumination, through both the table and crown bezel facets, one should have no problem seeing the double images.
 

 
Needless to say, for some reason, samples of C3's synthetic moissanite were unavailable to the author at the AGTA show. Perhaps it is because I have been highly critical about the scare tactics being used to market the synthetic moissanite tester being hyped by C3, Inc. as "the" solution. By the way it seems you can only buy a sample of synthetic moissanite at this time,  IF you have paid $500 dollars or so for a moissanite "tester". UPDATE  (Product is now available from C3 and for sale without buying the $500.00 detector) The following is my commentary regarding the article published in Gems & Gemology regarding synthetic moissanite, also know as TestoriteTM
 
TestoriteTM Update
GIA  published in the Winter 97 Gems & Gemology an article on TestoriteTM aka moissanite which may have, in my opinion, presented unknowingly, misleading information regarding the characteristics of synthetic moissanite. In Table 2 GIA
indicated that approximately 50% of the samples tested were electrically conductive, based on testing with an old GIA
conductometer, which I believe is akin to a wheatstone bridge device. They also indicated that the samples tested were "all"
6H polytype silicon carbide. (This turns out to be true)

Opinion:

It is my professional engineering opinion that one of two things happened.. the sample set GIA tested were of differing
polytypes (differing crystal constants and or dopings) and/or that the "set point" on the conductometer was too low to
"indicate" conductance. (The latter was true).

Addenda.. The GIA conductometer was simply a 115 volt isolation transformer in series with a meter and the stone.

I believe that the GIA conductometer was designed to measure electrical conductance of type IIb diamonds (0.1-10 ohm-meter resistivity) vs 10^14 ohm-meter for Type Ia diamonds. According to GIA, they did not know the "set" point of the GIA conductometer. According to data posted on the Cree Research web site (http://www.cree.com), they produce moissanite, some of which has electrical conductivity on the order of 400 ohm-meter or less and a semi insulating type having
conductivity greater than 10^7 ohm-meter.

Why is this important.(All previous sarcasm relating to what one of the authors of the Gems & Gemology article should and probably did know are now removed... such thin skins)

Electrical conductivity has been proven a reliable method of detecting synthetic moissanite, once the facts were known. Only the manufacturer of the Moiskeeteer has kept pace with the significant advances made by Cree in the production of synthetic moissanite and has produced significant advances in detector technology. I have had the priviledge of working with them and will announce the newest developments in about a weeks time.. Delivery of this new detector technology will be late October.
 
 

Silicon Carbide Gemstones

The following are the patent claims from US patent 5723391:

Patent 5723391 Claims:
1 A finished synthetic silicon carbide gemstone comprising a single-crystal of synthetic silicon carbide having facets polished to a degree sufficient to permit the introduction of light into the gemstone for internal reflection from inside the gemstone.
2 The finished synthetic silicon carbide gemstone of claim 1 wherein the synthetic silicon carbide has a crystalline structure selected from the group consisting of 6H SiC and 4H SiC
3) A simulated diamond gemstone comprising a single crystal of colorless synthetic silicon carbide having facets polished to a degree sufficient to permit the introduction of light into the gemstone for internal reflection from inside the gemstone.
4 The simulated diamond gemstone of claim 3 wherein said facets are characteristic of a diamond cut
5) The simulated diamond gemstone of claim 4 wherein said diamond cut is a round brilliant cut.
6) The simulated diamond gemstone of claim 3 wherein the synthetic silicon carbide has a crystalline structure selected from the group containing 6H SiC and 4H SiC.
7) The simulated diamond gemstone of claim 3 wherein the synthetic silicon carbide is intrinsic silicon carbide.
8) A finished synthetic silicon carbide gemstone having a color comprising a single crystal of synthetic silicon carbide containing dopant atoms at a concentration sufficient to produce a visibly discernible color, said gemstone having facets polished to a degree sufficient to permit the introduction of light into the gemstone for internal reflection from inside the gemstone
9) The finished silicon carbide gemstone of claim 8 having color crystalline structure and doping characteristics selected from the group consisting of...

Blue                    6H SiC   Al doped
Purple . .             6H SiC   high Al doped
Purple               24H Sic   N-doped..
Green                 6H SiC   N doped
Yellow                3C SiC  undoped
Yellow Green     3C SiC   N doped
Red                  27H SiC   N doped
Light Brown       4H SiC   Low N doped
Yellow Orange   8H SiC   N doped .

10) The finished synthetic silicon carbide gemstone of claim 8 wherein said dopant atoms are present in the crystal of synthetic silicon carbide at a concentration in the range of 10^15 to 10^19 carrier atoms per cubic centimeter.
11) The finished synthetic silicon carbide gemstone of claim 9  wherein said dopant atoms are present in the crystal of synthetic silicon carbide at a concentration in the range of 10^15 to 10^19 carrier atoms per cubic centimeter.
12) The finished synthetic silicon carbide gemstone of claim 8 wherein the synthetic silicon carbide has a crystalline structure selected from the group consisting of 6H SiC and 4H SiC.
13) A simulated diamond gemstone comprising a single crystal of colorless synthetic silicon carbide having facets polished to a degree of smoothness characteristic of finished diamond gemstones.
14) The simulated diamond gemstone of claim 13 wherein said facets are characteristic of a diamond cut
15) The simulated diamond gemstone of claim 14 wherein said diamond cut is a round brilliant cut.
16) The simulated diamond gemstone of claim 13 wherein the synthetic silicon carbide has a crystalline structure selected from the group consisting of 6H SiC and 4H SiC.
17) The simulated diamond gemstone of claim 16 wherein the synthetic silicon carbide is intrinsic silicon carbide


 

Russian Synthetic Moissanite

Shown below are the optical spectra of two samples of silicon carbide produced in one of the former Russian republics. This material was of a grayish yellowish green hue and full of micropipes.. The optical effect of the micropipes, which form parallel to the C-axis of the crystal, was accentuated by the cutting of the stones, both having tables apparantly parallel with the C-axis as opposed to C3's optimal cutting of the tables perpendicular to the C-axis, which minimizes the effect of the micropipes as well as double refraction effects.
 
 

 
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