illimanite is named after Benjamin Silliman, Professor of Chemistry and Geology, Yale University, New Haven, Connecticut, USA.
Sillimanite forms transparent, rather slate-blue or blue-green slender poorly terminated prismatic crystals, and some material is fibrous.
Violet–blue stones have been reported from Mogok Stone Tract, Myanmar and greyish green chatoyant material is found in the Sri Lanka gem gravels.