Dumortierite: Gemstones Information Dumortierite is Al7(BO3)(SiO4)3O3 and forms fibrous or columnar crystals in the orthorhombic system with, in general, poor development of faces. Ornamental dumortierite occurs most commonly in blue or violet masses, faceted examples being quite rare due to the scarcity of individual crystals. The hardness of the massive material is 7; some crystals have hardness over 8. The SG is 3.21–3.41. The RI would show as a blurred shadow edge on the refractometer at about 1.68; transparent specimens are small but might show birefringence which may be in the range 0.15–0.37.A red-brown faceted Sri Lankan specimen of dumortierite had SG of 3.41 and RI 1.686, 1.722, 1.723, biaxial negative with birefringence 0.037,pleochroic colours black, deep red-brown and brown. The hardness was 8. Dumortierite occurs in pegmatite veins or in aluminium-rich regionally metamorphosed rocks. Ornamental specimens are mainly from Sri Lanka or Madagascar. Quartz impregnating dumortierite has been found in Arizona, USA.