Kornerupine

Crystal system Orthorhombic
Transparency Transparent
Luster Vitreous
Fracture Conchoidal
Cleavage Perfect prismatic
Specific Gravity 3.30
Hardness 6.5
Optical Character Biaxial - ; Double Refractive
Refractive index 1.667-1.680
Birefringence 0.013
Dispersion 0.019
Fluorescence None
Pleochroism Strong to distinct; green & yellow to red brown
Chemical Formula MgAl2SiO6
Comments Attacked by acids
Streak

Kornerupine (korn-neh-R00-peen) was first identified by the Danish geologist, A.N. Kornerup, after the mineral's discovery in Greenland. Later, gem quality material was found in Sri Lanka, Madagascar and Mogok, Burma. It is basically magnesium-aluminum silicate, corresponding to the formula MgAl2SiO6. It also contains small amounts of alkali metals, plus boron and a hydroxide ion. In a pure form it would be colorless, but the color is usually brown, yellow or green. The dark-brown stones from Sri Lanka are most frequently seen in collections.

This rarely used gem mineral crystallizes in the orthorhombic system and also occurs in aggregates resembling sillimanite. It has a hardness of 6 1/2, a vitreous luster, and a specific-gravity range of 1.27 to 3.44, with gem material usually about 3.32. The indices are usually near 1.665 - 1.677, with a very strongly negative sign, in that the middle index is only .001 below the high index. However, the low reading varies from approximately 1.661 to 1.682 and the high from 1.674 to 1.699. The birefringence varies from approximately .013 when the indices are low to about .017 when they are high. The color dispersion is .019. Green and yellow to reddish-brown dichroism is a characteristic feature. Because of this distinctive pleochroism and its strongly negative sign, kornerupine is seldom difficult to identify.

When fashioning kornerupine, the material should be oriented for the most attractive color. 43° and 39° are usually used for the crown and pavilion angles, respectively, and Linde A power on a tin lap results in a satisfactory polish.