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Andalusite (and a LOO site), silicates class; andalusite group
Chiastolite; a variety of andalusite
Chemical formula: Al 2 SiO 5 , aluminum silicate. Kyanite and sillimanite have the same chemical composition as andalusite, but different crystal structures.
RI: 1.63-1.65 Cleavage: good in one direction, poor in one direction
Birefringence : .007-.013 Hardness: 6.5-7.5; chiastolite 5-5.5
Luster : vitreous, greasy SG: 3.13-3.20 (lower for chiastolite )
Dispersion : 0.016 Optic char: DR biaxial negative; chiastolite: AGG
Fracture : conchoidal Crystal system: orthorhombic
Toughness : Fair to good Crystal shape: slender prisms, waterworn pebbles, massive
Absorption spectrum : not diagnostic, but may show clusters of fine lines around 485 nm to 518 nm, and around 550 nm.
Pleochroism : strong with two or three pleochroic colors in green and orange stones-typically yellowish green, reddish brown, & yellow or green or colorless.
Fluorescence : inert to LW; may fluoresce weak to moderate green to yellowish green in SW.
Treatments : normally none. It can be heat treated to improve its color, but this is seldom done .
Stable to light; stable to heat unless liquid inclusions are present; no reaction to chemicals.
Ultrasonic cleaning is usually safe, steamer is risky if liquid inclusions are present.
Sometimes called the "poor man's alexandrite," andalusite often displays two distinct colors face up, which are usually yellowish green and orange. A third yellow color may also be visible. Unlike alexandrite, whose colors change when the lighting is switched from daylight to incandescent lighting, andalusite typically shows two colors simultaneously under the same light. This is because cutters generally orient andalusite to maximize its strong pleochroism, the property of certain minerals to exhibit different colors when viewed from different directions. The contrasting colors create a distinctive looking gemstone. Gem cuts with a long axis such as an oval or rectangle tend to show one color near the center and a second color near the ends; round cuts usually blend the colors into a mosaic. Occasionally, andalusites are cut to emphasize just their orange or pink color .
Andalusite is named after Andalusia, an autonomous community in Spain where it was first discovered. A translucent variety that has graphite inclusions forming a cross is called chiastolite. The name is from the Greek chiastos , meaning "arranged diagonally" because the pattern of carbon inclusions resembles the Greek letter chi, which is written "ϰ." Chiastolite is sometimes cut as amulets in countries such as Spain, where the symbol of the cross has deep religious significance. Because of impurities, chiastolite may have a lower hardness and density than transparent stones.
Most gem-grade andalusite is from Brazil in the states of Minas Gerais and Espirito Santo. Sri Lanka and Myanmar are minor sources. Chiastolite is mined in China, South Australia, Spain, Siberia, Myanmar, Zimbabwe, and California, Pennsylvania and Maine in the U.S. In recent years, large quantities of excellent chiastolite have been coming from Hunan province, China.
Despite its rarity and unique appearance, prices of andalusite are relatively low. Retail prices for extra fine andalusite above five carats seldom surpass $500/ct. Low-quality material less than three carats may sell at prices below $20/ct. The retail range of eye-clean andalusite about one carat in size is roughly $50/ct to $300/ct.
According to dealer Simon Watt, the finest andalusite would be orange-brown and olive-green with flashes of pink, but preferences vary. The tone (lightness / darkness) and saturation (intensity) is more important than the actual hue (e.g., orange, yellow, green, pink, etc.). Very dark or very light stones generally cost less than more colorful ones in medium to medium-dark tones. Unlike gems such as sapphire, where pleochroism may be a negative factor, it is desirable for andalusite to display more than one color in the face-up position. Clarity and transparency are important factors that can determine whether a stone is industrial, commercial, or fine quality. Highly included material can sell for less than $5.00 per carat. The quality of the cutting also affects price because good cutting requires more time and usually results in a greater loss of weight from the rough. But it is worth the additional cost because the result of proper cutting is greater brilliance and a better display of color.