Class : Silicates
    Subclass : Nesosilicates
    Crystal system : Orthorhombic
    Chemistry : Al₇(BO₃)(SiO₄)₃O₃
    Rarity : Rare

Dumortierite is a rare mineral present in different rocks (granulites, pegmatites and quartzite from Madagascar, garnet gneiss from France), having in common the presence of boron which suggests high temperature magmatic emanations (pneumatolites). It owes its name to the French paleontologist Eugène Dumortier. It generally occurs in compact fibrous or fibroradiated aggregates, in columnar masses, rarely in distinct prismatic crystals, of a magnificent cobalt blue to lavender blue color. It is a locally important mineral for the manufacture of refractories and ceramics (Oreana in Nevada, Minas Gerais in Brazil). It is also known to play an ancillary role as an insulating material and sometimes as a fine or ornamental stone.

In France, we should point out the type locality of Beaunan (Rhône) where dumortierite appears in the form of microscopic blue-violet fibers in a pegmatite vein inserted in a micachist.

Twinning is common on {110}, sometimes producing trills.

Quartz crystals containing dumortierite fibers were able to see some of their faces repolished in order to improve the visibility of their inclusions.

False quartz geodes in which blue plastic needles have been glued are currently circulating on the market (photo on the left), caution... See our article on the subject here : fake dumortierite geodes