Danburite: Gemstones Information
Danburite, most often colourless or pale yellow (orange specimens may have been treated), is calcium borosilicate CaB2 (SiO4)2. Crystals of the orthorhombic system are prismatic with rhombohedral to square crosssection. The hardness is 7–7.25 and the SG 3.00. RI for the alpha, beta and gamma rays is 1.627–1.633, 1.630–1.636, 1.633–1.639 with birefringence 0.006–0.008. As the angle between the optic axes is nearly 90°, danburite is optically positive for blue to violet light and negative for red to green light. Some danburite may show the fine line absorption spectrum of RE.Under LWUV danburite will glow a bright sky-blue to blue-green. Red thermoluminescence has been reported.Danburite occurs in granites and in metamorphosed carbonate rocks associated with hydrothermal activity. Specimens suitable for fashioning come from Sri Lanka, Myanmar (in particular, yellow material), Mexico,where a light pink variety has been reported; Madagascar, where yellow crystals are found at Mt Bity and from Russia. Colourless crystals have been found at Obira, Bungo, Oita Prefecture, Japan; some have been faceted. Danburite will float in di-iodomethane while topaz will sink. In bromoform (tri-bromomethane), whose use is subject to restrictions, similarly coloured citrine will float.