A synthetic scheelite was introduced in 1963. It is essentially a calcium tungstate produced by the Verneuil process, and was initially employed for industrial purposes. At later dates it was reported to be manufactured by the Czochralski pulling technique in the same manner as yttrium-aluminum garnet.
Natural scheelite commonly occurs in brown and yellow, but synthetic scheelite is primarily colorless, but may be "doped" with various impurities to give the desired colors. The colored materials show distinctive absorption spectra, the most striking of which are those due to rare metals such as didymium. In conjunction with the varied spectra, different fluorescence under both long-wave and short-wave ultra violet is also noted.
The refractive indices of synthetic scheelite are above the limits of the refractometer, and are calculated to be 1.920 to 1.936, with a birefringence of about 0.016. Scheelite is tetragonal, and thus uniaxial; its optic sign is positive. Its specific gravity is 6 and it has a hardness of 5 on the Mohs scale. Inclusions in the synthetic are rarely present, but when noted they are the typical spherical elongated gas bubbles found in numerous synthetic (Figure 36) or inclusions of rhodium or iridium (Figure 37 and 38).
These several beautiful small crystals had a color change reminiscent of light colored natural Russian alexandrite, and were free from inclusions. The materials fine transparency can be noted in Figure 39.
Today, flux-grown synthetic alexandrite is produced in commercial quantities by Creative Crystalsl Inc. Danville, California. The refractive Indexes are 1.746 and 1.755, and the specific gravity is 3.73. Veillike inclusions typical of flux-grown synthetics may be seen. Tubes of flux, parallel planes of flux inclusions, hexagonal or trapeze-shaped metallic platelets and growth lines may also be found in synthetic flux alexandrite.
Alexandrite has also been synthesized by the Czorchralski pulling method. Its refractive indices are 1.740 to 1.749. The specific gravity is 3.715. Lath-shaped included crystals, needlelike inclusions and curved striae may be present in this synthetic.
A synthetic alexandrite called, "Crescen Vert Alexandrite" is
presently being manufactured by Kyoto Ceramic, lnc., in Japan.
It is believed to be produced by a "pulling" technique. This synthetic has a change of color from greenish blue to purplish red. Its refractive indices are 1.741 to 1.748; birefringence 0.007; specific gravity about 3.70. The material fluoresces an intense red under both long-and short-wave ultraviolet and curved striae sometimes occur.
In 1970, experts analyzed a synthetic material which tested
to be turquoise by X-ray diffraction. Synthetic turquoise manufactured by Pierre Gilson was first marketed in 1972. Research on "Gilson Created Turquoise" has shown that it is a true synthetic. The properties of Gilson synthetic turquoise are as follows : refractive index approximately 1.60, specific gravity 2.62 to 2.67, and may be as high as 2.75. Hardness is 5 to 6.
This synthetic has a characteristic appearance under 50x
magnification. Tiny dark blue microspheres are visible in a light-
colored groundmass. It is produced in a medium blue and an intense blue color. Both shades of blue may have matrix.
A true synthetic opal with black or white background was developed by Pierre Gilson in 1972. The refractive index of synthetic opal is 1.44 and the specific gravity is 2.05 ( +0.03). Under magnification Gilson synthetic opals have characteristic patterns that are not seen in natural opal. Synthetic opal is more transparent to ultra violet radiation than natural opal. Natural white opal phosphoresces for a longer period of time than synthetic white opal when exposed to ultraviolet light.