Russian Gemstones Encyclopedia

Vladimir Bukanov. Russian Gemstones Encyclopedia



DUMORTIERITE (Dumortierit—Dumortiérite—ƒюмортьерит) (Gonnard, F. 1881), after Eugéne Dumortier (1802-1873), Fr. paleontologist.

Composition & Properties. Silicate – Al6(Al,Mg,Fe3+,)[(O,OH)3|BO3|(SiO4)3], subclass orhosilicates, orthorhombic system. Hardness 7-8.5. Density 3.3-3.4. Glass luster, silk one in aggregates. Cleavage in one direction. Fragile. Dumortierite is found as columnar, needle to hair-like crystals and their twisted-fibrous aggregates. The length of crystals is up to 5 cm., the thickness is not more than 1 cm., and for hair-like crystals – 1x1.5 cm. Color: light blue to bright blue, greenish-blue, red, violet, greenish-yellow. Transparent to translucent. It displays clear pleochroism from cobalt-blue to light violet to colorless.

Deposits. Dumortierite is found in pegmatites, metamorphic rocks and hydrothermal quartz veins. In Russia, transparent crystals of dumortierite were discovered in pegmatites of the Mokrusha mine, in the Middle Urals. In Kazakhstan, in North. Pribalkhashia, at the Zhanat deposit, in the Kounrad region, dumortierite is found as veins and bump of bright blue, light blue and violet color in quartz-sericite rocks. To the northwest from Kounrad, andalusite-dumortierite quartzite with the content of dumortierite more than 70% are developed; they can be used alas well as ornamental stones. Another Kentobe deposit with dumortierite in the form of concretions up to 40 cm. in diameter is situated in the region of Karkalinsk. Another one deposit of the same type with dumortierite is Bes-Choku in Central Kazakhstan. In Armenia, dumortierite is found as blue sphaerolites in secondary quartzites of the Lalvar Mt., they can be classified as ornamental stones. In Sweden, at the manganese Långban deposit they found veined quartz of rose hue because of inclusions of dumortierite. In France, dumortierite was discovered at the Beaunan deposit, near Lyon, Rhone Dept. In Italy, its blue-violet needles up to 3 cm. in pegmatite in Val Schiessone, Lombardy, are known. In Africa, crystals of dumortierite were found in pegmatites of Namibia, at the deposits Etemba, Omaruru, and Karasburg (Kalkfontein); in South Africa – in the Northern Cape Prov; and also in Mozambique. On Madagascar, at the Sahatany deposit near Anatandrokombi, they found gem-quality crystals of dumortierite 6x0.6 cm. in size. On Sri Lanka, at placers there were separate findings of transparent brown and red dumortierite. In India, dumortierite exists in quartz veins of the region of Mogra, near Bhandära, Mahäräshtra.

In the U.S.A., in Nevada, at the deposits Rochester and Oreana, Pershing Co., they discovered veined quartz of rose color because of inclusions of dumortierite. In California, they found it at the Dehesa mine, San Diego Co. However, because of rarity of findings and small size, crystals of dumortierite have only collection significance. Interesting deposits of ornamental dumortierite are known also in the states of Arizona, Washington, New Mexico and Montana. In Brazil, transparent crystals of dumortierite were discovered at one of the deposits of Minas Gerais State. They produce faceted stones 1-4 ct. of perfect light blue color from there crystals. In Bahia State, dumortierite was discovered at the Boquira mine and in quartzites of the Serra de Vereda deposit. In Mexico, dumortierite is extracted in the San Luis Potosí State, near Guadalquiwir, at the Guadalkasara mine.

Treatment. After faceting, dumortierite is rarely more than 2 ct. Because of its high hardness, it is competing successfully with azurite and lazurite. As an ornamental stone, veined quartz, or quartzite with blue spots, enriched with inclusions of needle aggregates of dumortierite is of special interest; it is called dumortierite quartz. This rock and others with inclusions of dumortierite are processed usually in cabochons, which display, sometimes, the effect of cat’s-eye – rose quartz cat’s-eye or dumortierite cat’s-eye. In cabochons such rock is very popular at the American stone market under the trade name rose jade.