Andalusite: Gemstone Information
Andalusite: Gemstone Information
Andalusite’s colours range from a greenish brown to a rich green incolour, the stones owing much of their attractiveness to their pleochro-ism which shows as red, yellow and green colours. The mineral is, with kyanite and sillimanite, a polymorph of Al2SiO5. While the crystals,members of the orthorhombic system, have a prismatic habit with vertically striated prisms which are nearly square in section and capped with pyramids, most gem material is found as water-worn pebbles.The hardness of andalusite is 7.5 on Mohs’ scale and the SG varies from 3.15 to 3.17. The least and greatest RIs vary from 1.634 to 1.641 and 1.644 to 1.648, the biaxial negative, DR varying from 0.007 to 0.011. The pleochroic colours are yellow, green and red which may vary in intensity.The absorption spectrum varies: a deep green variety, believed to come from Brazil, and in which Mn occupies the octahedral Al site, shows a manganese spectrum with an absorption band, graded in intensity, which ends in a knife-sharp edge at 553.5 nm and which is accompanied by fine lines at 550.5 and 547.5 nm and fainter ones at 518 and 495 nm.
There is a strong absorption in the blue and violet but a band at 455 nm can just be seen. This has been noted in the normal types of gem and alusite and these stones, particularly those from Sri Lanka, show this band to be accompanied by a narrow band at 436 nm. These last two bands are probably due to iron. Andalusite exhibits no luminescenceunder LWUV but the brownish greenstones from Brazil often show adark green to yellowish green glow under SW and greenish yellow under X-rays.While andalusite of a dull green is found in the gem gravels of Sri Lanka, Brazil is a more common source of the gem material, wherethe stones are found in secondary deposits, either on the stream bedsor on the slopes of the hills under several yards of clay and gravel. This is in an area some 15 km wide and 40 km in length near the town ofSanta Tereza in the state of Espirito Santo. Andalusite is also found in the state of Minas Gerais. The gem mineral is rarely found in crystals with good form and most of the gemmy material is found as water-wornpebbles. The Brazilian stones usually show a very strong flesh-red and olive-green pleochroism, which is especially well brought out when the stones are cut with the table facet nearly at right angles to the vertical axis of the crystal.
Such stones may closely resemble certain types of Brazilian tourmalines. A dark green andalusite is also found in Braziland so too are stones of a rose-red colour, although these are rare.The opaque variety of andalusite in which the crystals, cut across theirlength, display carbonaceous inclusions in the form of a cross is known as chiastolite. Sections are cut and polished for amulets and charms inseveral countries, and particularly in the Pyrenees. Owing to impuritieschiastolite may have a lower hardness and SG than transparent crystals.Chiastolite crystals are found in the Nerchinsk district of Transbaikalia inSiberia, at Sallas de Rohan near Brieux in Brittany, France, and Mount Howden, north of Bimbowrie in South Australia. Other localities are at Hof in the Fichtelgebirge, Bavaria, Germany, and in the slates around Skiddaw in Cumberland, England, but here the needle-like prisms are too slender for fashioning into gems. Sources in the USA are Arizona, and Madera in Madera County, California. Other localities are in Maine,Connecticut, Massachusetts and New Mexico, and Fannin County,Georgia. Good examples of the chiastolite variety are found in the Chantaing U range, near Kyankse, south of Mandalay, in Myanmar and Zimbabwe.