Balitsky VS1, Makhina IB2, Mar’in AA2, Dorogovin BA2, Shigley JE3 and Rossman GR4

1Institute of Experimental Mineralogy RAS, Russia
2All-Russian Scientific Research Institute for Mineral Raw Materials Synthesis (VNIISIMS), Russia
3Gemological Institute of America, USA
4California Institute of Technology, USA
 

The first commercial synthetic ametrine from Russia and its gemmological characteristics

    In Russia (at IEM RAS and VNIISIMS) an industrial technology has been developed growing single crystals of bi-colored amethyst-citrine quartz which are analogous to the natural ametrine found in a deposit in Bolivia (Vargas & Vargas, 1980; Vasconcelos et al., 1994). The technology is based on the results of previous studies of the origin of amethyst and citrine colorings and the effect of different physical-chemical and growth factors on their formation, stability and the character of color distribution in the crystals (Balitzky, 1980; Nassau, 1981; Balitsky & Balitskaya, 1986).

    The first industrial batch of crystals (of the order of 100 kg) was obtained in 1994. Similar to the crystals of synthetic amethyst and citrine, ametrine is grown in autoclaves from strongly alkaline potassium carbonate solutions in the presence of ferric ions and an oxidizer under the temperatures from 350 to 390 degC and pressures up to 1.5 kbar. In one autoclave with the volume of one thousand liters, 250 - 300 kg of ametrine crystals can be grown over a period of 2-3 months. The weight of individual single crystals varies from several grams (the most valuable faceting material) up to 1 kg. In 1995 - 1997 the crystals of synthetic ametrine were sold at prices (depending on the quality and sizes) from 180 to 300 US$/kg.

    As is the case for crystals of natural ametrine, the synthetic analogues are characterized by sector zoning, which results from a selective distribution of amethyst and citrine colorings in dif-ferent zones of growth. Crystals grown on seeds parallel to the pinacoid {000l}, as is shown in Nassau (1981), are characterized by the simplest distribution of colors. The identification of gemstones made from such crystals is performed rather simply proceeding from the peculiarities of their internal structure and the distribution of colors in growth sectors of the rhombohedrons and other faces. However, the technology now available allows one to grow more than 12 varieties of crystals, including the crystals described earlier (Nassau, 1981). The internal structure and distribution of the amethyst and citrine colors in several of these varieties is more complicated and closer to natural ametrine. The identification of stones faceted from such crystals requires the identification of all the distinguishing features of synthetic amethyst and citrine and is not always easily accomplished.

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BALITSKY, V.S. & BALITSKAYA, LV. (1986): Phys. Chem. Miner. 13, 415-421.
NASSAU, K. (1981): Gems & Gemology 17, 37-39.
VASCONCELOS, P.M., WENK, H.-R. & ROSSMAN, G.R. (1994): Gems & Gemology 30, 4-23.
VARGAS, G. & VARGAS, M. (1980): Lapidary Journal 34, 1504-1506.