Anorthite: Gemstone Information
Anorthite is usually found in contact metamorphic limestones and as a constituent in some basic calc-alkali plutonic rocks and lavas. Notable localities include USA (Alaska; California; Franklin, New Jersey; Nevada); Italy (Trentino, the lavas of Vesuvius and Monte Somma); Finland; Sweden (Södermanland); India (Tamil Nadu); and Japan (Miyake – very rare, red transparent crystals have been facetted as gems).
Gems are rare and normally only cut for collectors. The Smithsonian has a colourless 1.58 ct anorthite from Nevada.
Crystals are translucent to opaque and only sometimes transparent. Lamellar twinning, which is very common, may cause a grooved effect on crystal and cleavage surfaces that appear as striations.
• Colour: Usually white, grey or colourless but can be pale shades of other colours. Occassionally reddish
• Lustre: Vitreous to dull
• Varieties: Anorthite-pargasite-ruby rock; colourless anorthite matrix 93–96%An – locality uncertain – Myanmar or Longido, Africa. Cut as cabochons. An atypical anorthosite, also used as a gemrock, comes from a locality recorded only as the Philippines (Johnson and Koivula, 1996). This rock is described as granular, overall white and containing sporadic green crystals up to 2.0 mm across with its granular material consisting largely of nearly pure anorthite plus minor interstitial oligoclase and crystals of zoned garnet, ‘primarily uvarovite with varying amounts of andradite in solution‘. It is said to roughly resemble jade.
• Crystal system: Triclinic
• Refractive index: α 1.572–1.576; β 1.578–1.583; γ 1.583–1.588 Biaxial ve
• Birefringence: 0.011–0.012
• Density: 2.72–2.75
• Hardness: 6–6.5
• Dispersion: Weak
• Cleavage/fracture: Perfect in one and good in another direction forming nearly right-angled prisms/brittle – conchoidal to uneven fracture
• Fluorescence: LWUV – cream, pale brown.