CHEMISTRY-- Al2SiO5 + Fe
CRYSTALLOGRAPHY-- Orthorhombic
REFRACTIVE INDEX --1.629 - 1.650
HARDNESS-- 6.5 - 7.5
SPECIFIC GRAVITY-- 3.13 - 3.17
CLEAVAGE-- Distinct one direction
HEAT SENSITIVE-- No
WEARABILITY*-- Very Good
SPECIAL CARE INSTRUCTIONS-- None
ENHANCEMENTS-- Can be heat treated to improve color. Rarely done.
*Wearability is graded as Excellent, Very Good, Good, Poor, and Forget It!Andalusite is a strongly pleiochroic gem, which means that is has different colors when viewed from different directions. While it is a strikingly beautiful gem, is largely unknown by the gem buying public... It's trichroic nature, which shows shades of brown, green and reddish brown depending on the orientation of the crystal, can be enhanced by specific orientation and cut. Those cuts with a long axis such as an oval, marquis or emerald cut tend to show one color near the center and a second, usually darker color near the ends. Square and round cuts usually blend the colors into a mosaic... Most specimens contain some inclusions, the most common being rutile needles. Brazil is the chief producer, but Sri Lanka, Russia and the US also have deposits. Of course, so does the site of original production, Andalusia, Spain. .. Andalusite is hard and tough enough for most jewelry uses. Poorly cut and polished stones are pretty dull and insipid looking, but a large, clean, well-cut Andalusite is a show stopper! A wholesale value on small, commercial quality faceted stones is at around $40 per carat for clean gems with good color. Larger stones and those with custom cuts fetch up to $200 per carat.
PleochroismStar Andalusite or Chiastolite
The differential selective absorption of the ordinary ray and the extraordinary ray in uniaxial stones (dichroism), or of the three rays corresponding to the three principal vibration directions in biaxial stones (trichroism). Of course, to the non-gemmologist, this explanation probably creates more confusion than clarification. It relates to the way light is refracted when passing through a stone. Typical colours for andalusite are green, brown, red, greenish-brown, brownish-green. Its pleochroic colours are red, green and yellow. Because of the pleochroism, the colour of an andalusite can be affected considerably by the direction in which it is cut.
An impure variety of andalusite known as chiastolite contains carbonaceous inclusions in a cruciform arrangement. These are cut and polished for use as amulets in many countries, largely because of the symbolism of the cross.