Kyanite: Gemstone Information
Kyanite with andalusite and sillimanite (both q.v.) is a trimorph of Al2SiO5.It occurs as characteristic bladed and tabular triclinic crystals which are often bent or twisted. Lamellar twining is common. The hardness is markedly directional, 5.5 parallel to the long direction and 7 at right angles to it. There is one direction of perfect cleavage which leaves a pearly lustre.The SG is 3.53–3.65 and the RI for the alpha, beta and gamma rays is 1.710–1.718, 1.719–1.724, 1.724–1.734, biaxial negative, birefringence 0.017. The colour is green to blue-green, frequently zoned, with pleochroism nearcolourless, violet blue and cobalt blue or different shades of green. Some bluish green kyanite may show an absorption line in the deep red at 701 nm and these and other specimens may also show two lines in the blue and a cut-off in the violet. Some dim red fluorescence may be seen under LWUV. Kyanite occurs in gneisses and schists and in granite pegmatites. The main sources of gem kyanite include Minas Gerais, Brazil, Machakos District, Kenya (some colourless specimens are reported as well as a fine blue; some striped crystals come from this source). Some dark blue specimens from Mozambique contain chromium and titanium. Facetable crystals have come from Yancy County, North Carolina, USA. A very fine deep sapphire-blue oval of 6.72 ct was shown to MO’D by Marcus McCallum in December 2004.