Quartz Windows in Chalcedony
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).
