Willemite: Gemstones Information

Willemite is Zn2SiO4 and forms both stout and slender prismatic transparent to translucent trigonal crystals with rhombohedral termi-nations or as coarse to fine granular masses. The colour ranges from colourless to white though yellow- and apple-green to blue-green or brown. The hardness is 5.5 and the SG 3.89–4.10. The RI for the ordinary and extraordinary rays is 1.691–1.694 and 1.719–1.725, uniaxial positive with birefringence 0.028. Willemite shows an intense green fluorescence under SWUV – some specimens may also show green phosphorescence. The absorption shows bands at 583, 540, 490, 442 and 420 nm, all fairlyweak, with a stronger absorption at 421 nm. Willemite occurs in zinc deposits in limestones and is an uncommon ore of zinc.  The ornamental value of willemite is in its fluorescent effects especially when combined with the fluorescence of red zincite and pink calcite; specimens with this effect are particularly associated with Franklin, New Jersey, USA.Facetable green willemite is most likely to come from Franklin though some blue-faceted stones have come from Mt Saint Hilaire, Quebec,Canada.