What is Kornerupine?
Kornerupine is a hard boron-bearing silicate that can show vivid green to blue-green colour and attractive pleochroism in gem-quality material.
Mineralogy and crystal structure
Reference formula: (Mg,Fe)9Al6(B,Al)Si5O21(OH). Kornerupine has an orthorhombic framework with channels and substantial Mg-Fe-Al-B substitution. Colour and pleochroism vary with composition, while the crystal structure is more reliable than visual similarity to tourmaline. Kornerupine is classified with silicates and has a orthorhombic crystal system. Typical hardness is 6.5–7 on the Mohs scale and specific gravity is approximately 3.2–3.4.
Colour, lustre and habit
Typical colours include green, blue-green, brown, yellow, violet. The usual prismatic, fibrous, granular habit and vitreous, pearly lustre help describe a specimen, but colour alone is not diagnostic.
Formation and notable localities
Kornerupine forms in or is associated with high-grade metamorphic rocks, boron-rich gneisses and metasomatic zones. Important documented localities include Sri Lanka, Madagascar, Greenland, Tanzania, Myanmar and Australia.
A collector gemstone with serious metamorphic depth
Kornerupine rewards close examination: green, blue and brown tones can shift with viewing direction, turning a single crystal into a moving colour study. Natural matrix pieces are especially appealing because they preserve the boron-rich metamorphic setting.
Collecting and care
Hard but brittle at edges; protect prismatic crystals and disclose treatments or orientation effects in cut stones.
