Sillimaite (named after professor Sillman of Yale University, USA) is blue to green, with distinct pleochroism showing pale yellowish green, dark green, and blue from different angles. When crystals occurs in long slender prisms in parallel groups, resembling fibres, the material is often called fibrolite.

Sillimanite is found in metamorphic rocks and occasionally in pegmatites. Blue and violet stones are found in Burma; greenish grey stones in Sri Lanka; fibrolite in Idaho (USA). Other sites include Czechoslovakia, India, Italy, Germany and Brazil.

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