Ametrine is a bicolored quartz variety that contains both amethyst and citrine zones in the same crystal. The only significant source of natural ametrine is eastern Bolivia’s Anahi mine, where it occurs in veins in a dolomitic limestone. The amethyst-citrine bicoloration results from quartz precipitation at very specific geochemical conditions, temperatures, and growth rates. The color of iron-bearing quartz depends on the valence state of the iron. The citrine color in Bolivian ametrine appears to come from the incorporation of very small aggregates of Fe3+. The amethyst color develops in two steps. First, individual Fe3+ions replace Si4+ ions in the quartz structure. To develop the amethyst color, the crystal must be exposed to ionizing radiation to oxidize the iron in the 4+ state.
Beginning in 1994, Russian gem-quality synthetic ametrine entered the market. Synthetic ametrine can be identified by employing a combination of techniques, such as visible inspection of twinning and color zoning, EDXRF chemical analysis, and IR spectra. In this article, Payette explains the possibilities for separating natural from synthetic ametrine with conventional gem lab equipment.
Quartz is a uniaxial mineral with two unique refractive indeces along its three crystallographic axes. The unique axis is the optic axis. The amethyst-citrine color boundary in natural ametrine is oriented roughly parallel to the optic axis; in synthetic stones, the boundary is oriented at an oblique angle to the optic axis. The gemologist needs only to find the direction of the optic axis to determine whether an ametrine is natural or synthetic. The optic axis in a uniaxial gemstone can be found with a polariscope that has a conoscope lens and, on occasion, with a refractometer.
The conoscope is a polariscope accessory tool. It is a strongly converging, strain-free glass sphere. When a gemstone is positioned between two crossed polarizers, interference colors that are centered in the specimen will be witnessed with the conoscope when the optic axis is exactly perpendicular to the polarizers.
Polariscope with a conoscope lens. The optic axis perpendicular to the polarizers. Illustration courtesy of Francine Payette.In this study, three types of natural and two types of synthetic ametrine were identified using a polariscope with a conoscope lens. Of the three natural stones, one was cut with its table perpendicular to the optic axis, one with its table parallel to the optic axis, and one with its table randomly oriented relative to the optic axis. One synthetic specimen was cut with its table parallel to the optic axis and the other with its table randomly oriented relative to the optic axis.
When a facet perpendicular to the optic axis of a uniaxial gemstone is measured with the refractometer, two constant readings will be observed while rotating the gem. If the facet measured is cut parallel to the optic axis, then the variable shadow will touch the constant shadow at one position during the rotation of the gem. This information enabled the identification of two of the natural samples and one of the synthetic ametrine samples with the refractometer alone. The direction of the optic axis could not be obtained by refractometer readings for the natural and synthetic ametrine samples cut with their table at random orientation to the optic axis, as those readings were not conclusive.
Separating synthetic ametrine from its natural counterpart using conventional gem lab equipment is possible, provided that the gemologist has a good understanding of the morphology and optical mineralogy of both natural and synthetic material.
Author : Guy Lalous