Sillimanite (named after Professor Silliman of Yale University, (USA) is blue to green, with distinct plcochroism showing pale yellowish green, dark green, and blue from different angles. When crystals occur in long slender prisms m parallel groups, resembling fibers, the material is often called liberality.

Sillimanite 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.

Tortoiseshell has now been largely replaced by plastic imitations.