Gems & Gemology, Winter 2016, Vol. 52, No. 4

Quartz Windows in Chalcedony

John I. Koivula
Chalcedony plate with quartz windows.
Figure 1. Measuring 52.56 × 36.71 × 2.88 mm, this half-moon-shaped chalcedony plate contains more than a dozen hexagonal quartz windows. Photo by Kevin Schumacher.

We recently examined a 55.08 ct polished half-moon-shaped plate of white and brownish yellow chalcedony from Madagascar that was fashioned by Falk Burger (Hard Works, Tucson, Arizona). As can be seen in figure 1, more than a dozen hexagonal windows of transparent colorless quartz accentuated by thin frames of brownish yellow chalcedony are randomly scattered throughout the host. In this specimen the quartz crystals would be considered protogenetic inclusions since the chalcedony formed around the preexisting crystals. The quartz crystals are all twinned on the Brazil law, and the c-axes of the quartz windows are all aligned in parallel fashion. As a result, when the chalcedony plate is examined between crossed polarizing filters, the transparent windows all display their twinning through the presence of colorful stellate patterns (figure 2) that vary in appearance as the plate is rotated or moved about in the polarized light field (see video at http://www.gia.edu/gems-gemology/quartz-window-chalcedony).

Brazil-law twinning in quartz windows.
Figure 2. When viewed between crossed polars, Brazil-law twinning in the quartz windows is revealed as stellate patterns (left). As the analyzer is rotated, different colors are revealed along the twinning (right). Photomicrographs by Nathan Renfro; field of view 19.01 mm.

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