Silicate of aluminium.
Crystal system Orthorhombic.
Appearance It occurs as squat prismatic crystals with poor luster, or aggregates of elongated crystals arranged in "sheaves." The color varies from light yellowish brown to green-brown, light brownish pink, grayish green, or definite green with pronounced pleochroism making it hard to identify the main color. Some crystals have dense, darker inclusions in the center which in cross section, exhibit a cruciform pattern. These stones are known as chiastolite Physical properties lt has a hardness of 7-7.5 and a density of about 3.13-3.20 g/cma. The refractive indices are about na 1.632, ﬂy 1.643. lt has distinct prismatic cleavage.
Genesis Andalusite is a characteristic mineral of areas of contact metamorphism between granites and argillaceous rocks. '
Occurrence It is not a rare mineral. Fine crystals are found in Spain (Andalusia), whence ‘the name. other sources are Brazil, the United States (California, Maine, Massachusetts), Australia, Sri Lanka, Burma, Austria.and occasionally ltaly.
This gem is named after Andalusia, where fine crystals occur.
Appearance It is a dull yellowish green with faint pink or brown reflections, or it may be yellowish brown-green or even violet brown-green or, rarely, a definite green. The difficulty in describing thecolor of this gem is because of its strong pleochroism, which is heightened by the facets of cut stones, _to_ the point of being almost iridescent; but the colors are never very lively. It has unexceptional, vitreous luster. It is regularly given a faceted, mixed oval cut; square or rectangular step cuts are"-also used, but thess are less suited to its modest luster and cannot do justice to its exceptional pleochroism.
Distinctive features Its peculiar color range, unusual pleochroism, and modest luster make it quite easy to distinguish from all other gems. Some say it can be confused with alexandrite, but the pleochroism of andalusite la very different from the true color change of the other. It is ture that when alexandrite is a yellowish green with a faint color change, it may look vaguely similar; but checking the density would immediately distinguish between the two stones. The chiastolite variety with its black cross-shape is unmistakable.
Occurrence The gem variety comes mainly from Brazil (state of Minas Gerais), but also Sri Lanka, Burma, the Soviet Union, the United States and, of course, Andalusia. Value Quite low, probably because of the rather drab color and poor luster. It is anything but a rare stone, yet it is not plentiful on the market and is mainly in demand with collectors. _The most valuable stones are those with the most pronounced greenish to reddish pleochroism. The chiastolite variety has a longer history of gem use than the transparent type, having been worn as an amulet, but its value is equally low and similar to that of opaque ornament tal materials.
Simulants and synthetics Not being valuable or well known, it is not imitated. Nor has it been produced synthetically, at any rate on a commercial scale.