Micro-World Gems & Gemology, Summer 2019, Vol. 55, No. 2

Quarterly Crystal: Dioptase in and on Quartz

Colorless quartz crystal with blue-green dioptase inclusions.
Figure 1. Measuring 25.28 × 17.51 × 7.67 mm and weighing 15.65 ct, this colorless quartz crystal from Namibia hosts numerous transparent blue-green inclusions of dioptase. Photo by Diego Sanchez.

Crystals of transparent colorless rock crystal quartz play host to a wide variety of mineral inclusions, some of which are highly attractive. Due to its transparency and bright blue-green color, one of the most visually striking inclusions occasionally found in quartz is dioptase. Occurring in the oxidized zones of some copper deposits, dioptase gets its vibrant color from copper, the chemical formula of dioptase being Cu6Si6O18•6H2O.

Recently we had the opportunity to examine an unusual arrowhead-shaped, fully terminated colorless quartz crystal that was decorated on one side with a crust of numerous trigonal transparent dioptase crystals that were both in and on their host. The geographic source for the crystal shown in figure 1is Kaokoveld, Namibia. Weighing 15.65 ct and measuring 25.28 × 17.51 × 7.67 mm, this unusual example of rock crystal plays host to numerous near-surface trigonal crystals (figure 2) up to 3.0 mm in length.

Trigonal blue-green dioptase crystals in and on quartz.
Figure 2. Trigonal blue-green crystals in and on the surface of this colorless quartz were identified by Raman analysis as dioptase. Photomicrograph by Nathan Renfro; field of view 11.75 mm.

The visual characteristics of the features—the trigonal micromorphology, degree of transparency, and color of the numerous inclusions—suggested dioptase; this was confirmed using laser Raman microspectrometry. The euhedral dioptase inclusions were situated on only one side of the quartz, which illustrates that they developed through directional deposition late in the quartz growth cycle.

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