Micro-World Gems & Gemology, Summer 2020, Vol. 56, No. 2

Quarterly Crystal: Pyrite in Quartz

Colombian quartz with pyrite inclusions.
Figure 1. This example of Colombian pyrite-containing rock crystal weighs 466.27 ct and measures 61.90 mm in length. Photo by Diego Sanchez.

Pyrite and quartz are two relatively common minerals. Transparent colorless quartz, known as rock crystal, is host to a wide variety of mineral and fluid inclusions, more so than any other gem mineral. While pyrite and rock crystal are both common, finding a good example of crystalline pyrite inside of rock crystal quartz is a gem mineral collector’s prize.

Modified octahedrons of pyrite in quartz.
Figure 2. These modified octahedrons of pyrite were deposited along one prism face of their quartz host. Photomicrograph by Nathan Renfro; field of view 7.62 mm.

Known for its fine emeralds, the Chivor mine in Colombia is also a mineralogical source of both pyrite and colorless quartz. On occasion, crystals of Colombian quartz are known to host pyrite. As shown in figure 1, we recently had the opportunity to examine one such specimen, which hosted numerous bright metallic brassy yellow modified octahedrons of pyrite (figure 2). The inclusions were not randomly scattered throughout the 466.27 ct quartz, but instead were situated neatly just under the surface of one of the prism faces. Another interesting feature was that all of the pyrite inclusions were present in a range of sizes with well-defined crystal faces, suggesting that the pyrite crystals precipitated syngenetically (at the same time) from a directional fluid rich in pyrite forming iron sulfide.

John I. Koivula is analytical microscopist, and Nathan Renfro is manager of colored stone identification, at GIA in Carlsbad, California.