|Transparency||Transparent to translucent|
|Optical Character||Biaxial - ; Double Refractive|
Blue stones: colorless, dark blue & violet-blue
Kyanite (KY-an-ite) is a seldom encountered mineral that is remarkable chiefly for its great variation in hardness: 4 to 5 in one direction and 6 to 7 at right angles. The alternate name, disthene (DIS-theen) is derived from two Greek words meaning "double" and "strength" in allusion to this unequal hardness. The mineral was formerly spelled "cyanite," but by international agreement the "k" spelling has been adopted. The name comes from the Greek word meaning "blue" since that is its usual hue. It may be light to dark blue, yellow, gray, green, brown or colorless.
Kyanite, sillimanite and andalusite all have the same chemical composition (Al2SiO5), but they are different minerals because they are different structurally. All three have been used in their impure forms as a source of porcelain for spark plugs. It is a triclinic mineral and almost always occurs in flat, bladelike crystals. It has one direction of perfect pinacoidal cleavage, a less perfect pinacoidal cleavage inclined at 74°, and a basal parting; therefore, the toughness is classed as fair to poor. The S.G. is approximately 3.56 to 3.68, the luster is vitreous to pearly, the fracture is conchoidal, and the streak is white. A characteristic of kyanite is its inclusions: flat, elongated fissures terminated at right angles by silver like cleavage cracks. The optic character is biaxial negative. Indices vary from about 1.712 - 1.728, rising to 1.716 - 1.731. The birefringence remains at about .015. Distinct light and dark-blue pleochroism is exhibited, but dispersion is weak (.011). A blue variety with a faint girasol effect has been called by the dubious name of "sapphire spar." Kyanite is not attacked by acids nor is it fusible.
Kyanite occurs principally in gneiss and mica-schist, accompanied by garnet and sometimes by staurolite and corundum.
Sources include Kashmir, Patiala and Punjab, India; Burma; St. Gothard, Switzerland; Russia; Brazil; Kenya, East Africa.
Separating this mineral from others that may resemble it in appearance, such as sapphire and topaz, is usually accomplished without difficulty by R.I. and S.G. No other mineral that resembles it has similar properties.
As might be suspected, this is a very difficult mineral to cut
and polish. Sawing is perilous, because it has a decided tendency to
split; trimming gently with small pliers is usually less hazardous.
It should be ground with the "grain;" i.e., parallel to the length
of the crystal. Great care and patience are needed. The same
precaution is advised for the polishing operation. Crown angles of
37° and pavilion angles of 42°, using Linde A on a tin lap, produce