Micro-World Gems & Gemology, Summer 2016, Vol. 52, No. 2

Chalcedony with Quartz Windows

Quartz windows in chalcedony
The trigonal windows of quartz found throughout this translucent white chalcedony displayed vibrant interference colors when examined between crossed polars. The black triangles in the lower left corner are also quartz. Photomicrograph by John I. Koivula; field of view 15.5 mm.

A translucent white chalcedony specimen containing a few small transparent quartz crystals was fashioned into a decorative polished plate by Falk Burger (Hard Works, Tucson, Arizona). One intriguing feature of this piece is that most of the transparent quartz crystals were aligned with their c-axes parallel to each other. When they were transected perpendicular to this axis during the polishing process, angular transparent windows were created, each displaying trigonal symmetry within their otherwise translucent chalcedony host. Interestingly, even though the white chalcedony was polished into a plate, it still showed a rather curious angular-looking three-dimensional texture. When examined between crossed polars, the triangular quartz windows ignited with vibrant interference colors, as seen in the accompanying image, due to Brazil-law twinning in the crystals.

John I. Koivula is the analytical microscopist at GIA in Carlsbad, California.