Colemanite

Colemanite is a hydrated calcium borate, Ca2B6O11·5H2O, and one of the most prized species among borate collectors for its glassy, often gemmy crystals and razor-sharp cleavage. It forms as a secondary mineral through the alteration of borax and ulexite in arid, alkaline lacustrine (playa) environments, typically alongside gypsum, calcite, celestite and other evaporite minerals. Crystallizing in the monoclinic system, colemanite develops in an unusually wide range of habits — short prisms, blocky crystals, spiky bladed groups, twinned individuals and radial nodules — a diversity of form rivaled among minerals perhaps only by calcite. Hardness sits at 4.5 on the Mohs scale, with a specific gravity of 2.42 and perfect cleavage on {010} that gives cut fragments their characteristic brilliant sheen.

The species was first described in 1884 from the Harmony Borax Works near Furnace Creek in Death Valley, California, its type locality, and was named after borax magnate William Tell Coleman. California remains the classic ground for fine specimens: the Kramer district at Boron (home of the historic U.S. Borax open pit mine) produces the well-known spiky bladed crystals and, more rarely, prized red and yellow colored pieces, while the Boraxo mine near Ryan and Twenty Mule Team Canyon in the Furnace Creek Mining District have yielded large, blocky, sometimes 15 cm single crystals. Nearby, the Calico Mountains near Daggett in San Bernardino County were an early 1880s source that helped launch the American borax industry. On the other side of the world, Turkey hosts the planet's richest colemanite reserves, with the Emet, Kirka, Bigadiç and Kestelek deposits in the Kütahya, Eskişehir and Bursa provinces producing exceptionally large, gemmy, colorless-to-ivory crystals — including individuals reported up to 30 cm — along with striking twinned and zoned examples from the Kestelek mine. Iran's Qarah Gol Boron Mine in Zanjan Province is another notable, if less common, source for collector-grade material.

Beyond the cabinet, colemanite is industrially important as a natural source of boron, historically the principal ore of borax until the 1930s and still valued for boric acid, borosilicate glass, fiberglass, ceramic glazes and fluxes. For collectors, though, its appeal lies in the contrast between the mundane industrial mineral and its finest crystallized specimens: lustrous, sometimes fluorescent, colorless-to-brown crystal clusters from Boron, Death Valley and Emet stand among the true classics of the borate mineral group, prized for their sharp forms, unusual twinning, and rarity in fine, well-crystallized condition.