Synthetic Quartz: A Designer Inclusion Specimen
As interest in gem and mineral inclusions grows, the value of inclusion specimens has increased as well. This has led to the relatively recent trend of simulated inclusion specimens being offered in the marketplace (see E.A. Skalwold, “Evolution of the inclusion illusion,” InColor, Summer 2016, pp. 22–23). To the best of this author’s knowledge, the synthesis of a quartz host with inclusions—or for that matter, any type of synthetic crystal—for the express purpose of creating a collectable inclusion specimen has not yet been reported and therefore presents a very interesting project to pursue.
Natural quartz plays host to a wide variety of inclusions, including several types of colorful garnets that often lend an aesthetic contrast to this already fascinating mineral. The author retained the services of a synthetic quartz manufacturer who refined and implemented her plan for growing four small specimens: one with pyrope garnets, one with almandine garnets, one with both types, and one without added garnets as control. The chosen garnets are brightly colored despite their tiny size and so fit with the desire to keep the finished quartz crystals small, given the long and expensive growth period required for the hydrothermal process.
Using a five-meter-tall industrial high-pressure autoclave, several runs were completed over a four-month period. Prior to the second run, the garnets were introduced into holes bored into the quartz. A few of the garnets were thus successfully captured and incorporated within the host as the second run continued. The nutrient solution for the quartz growth consisted of approximately 10 wt.% of Na2CO3 in pure water, with many trace elements originating from the milky vein Arkansas quartz used as the silica source. To produce the desired crystal morphology, a seed with “c-a” cut was used to initiate growth vertically along the c-axis and elongation along the a-axis. Rather than being hung by wires in the autoclave, the growing crystals sit on a shelf, and hence there is no wire in the finished specimen. The growth temperature was approximately 350°C in a pressurized environment of 700-plus bars.
When the autoclave was opened at the end of four months, four crystals of approximately the same size emerged intact, one of which is described here as representative of the entire set (figure 1). Along with “breadcrumb” inclusions familiar to gemologists, the suite of captured garnets was surrounded by unidentified white masses and radiating cracks. Quartz’s structure can be thought of as an open yet distorted framework of silicon and oxygen atoms. Because these bonds have angles that change rapidly with temperature, the volume of quartz changes rapidly with change in temperature—much more rapidly than the rather closely packed atoms in garnet. So it is not surprising that as the specimens cooled, the quartz shrank faster than the garnets, causing the quartz to fracture (figure 2). Having formed previous to the growth of the quartz that later captured them, these garnets would be considered “protogenetic” inclusions. Some liquid and gas originating from the autoclave’s environment was also captured as a two-phase inclusion running perpendicular to the c-axis of the quartz. The glassy prism faces of the crystals display characteristic diagonal striations, unlike the prism faces of natural quartz, which have horizontal striations (i.e., perpendicular to the c-axis). Originally, one of the four crystals was intended to be cut into a cabochon to illustrate a classic “rough and cut” suite, but it would have been a shame to sacrifice even one of these pristine and arguably unique synthetic quartz inclusion specimen simulants. Therefore, they will remain as they are in the author’s collection—as her own “designer inclusion specimens.”