Howlite: Gemstone Information
Howlite is Ca2B5SiO9(OH)5 and while found as white monoclinic tabular crystals, most ornamental material is nodular with H 3.5 and SG 2.53–2.59.Howlite is opaque white and may be porcellanous. The hardness is 3.5 and the SG 2.53–2.59. The RI for the alpha, beta and gamma rays is 1.583–1.586,1.596–1.598, 1.605 biaxial negative with a birefringence 0.022 but in practice a value of about 1.59 may be obtained. For ornamental purposes howlite is dyed to give an imitation of turquoise. Luminescent effects for the undyed material include a deep orange under LWUV for some Californian specimens and a brownish yellow under SW for some other material. Howlite is found in borate deposits, most ornamental pieces coming from various Californian locations.