Rough dumortierite from Madagascar

Dumortierite was named after the French scientist, M.E. Dumortier. Dumortierite is best known in its massive form, making an attractive violet and blue decorative gemstone when polished. Reddish brown and red varieties also occur. Prismatic crystals bigger than 1mm are very rare.

Demortierite is also found intergrown with rock crystal (colourless quartz) and is then called dumortierite quartz. This material is generally cut in cabochon or polished to make decorative stones.

Most gem-quality material is found in Nevada (USA). Other localities include France, Madagascar, Norway, Sri Lanka, Canada, Poland, Namibia and Italy.

Chemical Composition :

Aluminium Borosilicate – Al7O3(BO3)(SiO4)3

Crystal System / Forms :

Orthorhombic System / Prismatic crystals, often massive, fibrous, granular

Cuts & Uses :

Generally opaque gemstones are available and are cut into cabochons. Transparent varieties are very rare.

Hardness : 8 - 8.5; massive varieties 7
Lustre : Sub-Vitreous
Magnification :

Parallel fibrous inclusions, crystals, fingerprints.

Optic Character :

Anisotropic, D.R.; Biaxial negative. Commonly Opaque.

Pleochroism : Strong pleochroism if transparent, otherwise unable to detect.
Refractive Index / Birefringence :

1.686 – 1.723 / 0.037

Simulants (with separation tests) :

Sugilite (structure), Chalcedony (structure), Fluorite (optic character, R.I., U.V. fluorescence)

Sources :

France, U.S.A. (Arizona), Sri Lanka, Brazil.

Specific Tests :

Most gem quality dumortierite is actually dumortierite in quartz and may give R.I. readings for quartz.

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