Dumortierite is a mineral with a hardness of 7 out of 10 on the Mohs scale of mineral hardness [?]. These Orthorhomibicly structured gems are made of aluminium borosilicate, their full chemical compound being Al7(BO3)SiO4)3O3.
Dumortierite is a variety of gemstone of basic aluminum borosilicate with an orthorhombic crystal system. It appears usually in columnar or fibrous, radiating aggregates, sometimes reddish brown, dark blue, violet-blue. On the rare occasions that dumortierite forms crystals, they are prismatic. Faceted or prismatic blue or violet samples are rare, due to scarcity of individual crystals.
It is very hard, heavy and has perfect cleavage. It is transparent to translucent to opaque, with vitreous to dull luster, and is pleochroic. It is insoluble and infusible.
It forms in coarse-grained, acid, igneous rocks. It occurs in metamorphic rocks rich in aluminum, in some pegmatites (coarse-grained rocks formed by very slow cooling of magmatic fluids in a chemically-rich environment), and in contact metamorphic rocks.
Commercial deposits in the USA are found at the Oreana and Rochester mining districts (Nevada), Dehesa (California), and in Arizona; and also in Lyons (France), Norway, Mexico, Canada, Malagasy and Brazil. It is also found in pegmatites at Sondalo (Sondria, Italy).
Misleading terms include "California lapis" for the mixture of blue dumortierite and quartz. Sometimes it is included in a variety of quartzite, which is called "dumortierite quartz," where crystals are impregnated into quartz, then cut cabochon.
It is often confused with azurite, lapis lazuli, and sodalite.
Dumortierite is named after a French paleontologist, Eugene Dumortier.
Dumortierite is used for the manufacture of aluminum refractories. It is also an ornamental stone and is frequently cut as a gem, sometimes used as an imitation of lapis lazuli.