Diaspore: Gemstones Information

Some examples of diaspore, AlO(OH), have been found to show a colour change from greenish in daylight to pinkish brown under tungsten light,though much paler than the similar effect seen in alexandrite. Stronger colours are visible when crystals are cut into thin sections. Diaspore forms platy crystals in the orthorhombic system with a perfect cleavage in one direction and a distinct cleavage in another. This makes fashioning quite difficult as the hardness is only 6.5–7. The SG is 3.2–3.5 and the RI 1.682–1.706, 1.705–1.725 and 1.730–1.752 for the alpha, beta and gamma rays. The birefringence is 0.048, biaxial positive. Absorption bands noted in some specimens were at 471, 463 and 454 nm in the blue.Diaspore may be formed by the hydrothermal alteration of aluminous minerals and may be a hydrothermal mineral in some alkalic pegmatites.Large gem-quality crystals have been found from the area of Mamaris,Yagatan, Mugla Province, Turkey, and this seems to be the source of allfaceted stones to date.