Photo of Dumortierite in Processed & Rough Form
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.
What is Dumortierite?
Dumortierite is a blue aluminum borosilicate mineral, often found in quartz. It's responsible for the blue color in some blue quartz and can range from deep blue to violet-blue. Named after French paleontologist Eugene Dumortier.
Where is it found?
Major deposits exist in Brazil, Madagascar, Russia, and Nevada (USA). Significant deposits also found in Austria and France. High-quality specimens come from Brazil and Madagascar.
How can I identify Dumortierite?
Look for fibrous blue inclusions in quartz, hardness of 7, and distinct blue color that can appear similar to sodalite or lapis lazuli. Often shows subtle color variations and sometimes appears in star patterns.
What are its uses?
Used in jewelry as cabochons and beads. Has industrial applications in ceramics and porcelains. High-grade material is used in engineering ceramics for its high heat resistance.
How does it differ from other blue stones?
Often confused with sodalite or lapis lazuli, but dumortierite typically shows fibrous structures and is harder. It's usually found in quartz, giving it different optical properties from other blue stones.
What determines its value?
Value depends on color intensity, clarity of the host quartz, and pattern of the dumortierite inclusions. Specimens showing distinct patterns or star effects are particularly valued.
The specific gravity [?] for Dumortierite is 3.28, its refractive index [?] is 1.69-1.72, and its double refraction [?] is 0.037.
History
Dumortierite is named after a French paleontologist, Eugene Dumortier.
Industrial Usages
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.
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