SAS2000: Developed,
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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.
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.
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
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.
SAS2000 Spectrophotometer
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