Gem-Quality Andalusite from Brazil

Abstract

MATERIALS AND METHODS From the parcel of rough, we randomly selected about 150 specimens for examination that ranged from ~0.4 to 3 g. We studied 15 samples in detail for their crystallographic features, surface markings, cleavage directions, and pleochroism with respect to crystallographic orientation. These samples exhibited a variety of colors: green, yellowish green to brownish green, brownish pink, and distinctly bicolored green/pink. In addition, we selected six samples of the pinkish brown to brownish pink variety chiastolite, which displayed distinct (sometimes partial) cross-shaped arrangements of dark inclusions. About 20% of the 150 specimens were chiastolite, and they ranged in diaphaneity from transparent to opaque. We selected 72 of the 150 specimens, of moderate commercial quality, for fashioning on the basis of their coloration, clarity, and projected yield. Of these, 18 were faceted (figure 2) and the other 54 were left preformed. The preformed stones were categorized into three color groups: green (17 specimens), brownish pink (14), and strongly pleochroic pinkish brown/yellowish green (23). On an additional seven specimens (figure 3), we polished the largest parallel surfaces so we could obtain RI readings and examine their internal features. Standard gemological tests—including color observations, SG measurements, fluorescence reactions, and visible-range spectroscopy—were performed on all samples. RI and optic sign were measured only on GEM-QUALITY ANDALUSITE FROM BRAZIL