Blue Moonstone

GIA illustrates light blue moonstone from Sri Lanka. In 2005 the GAAJ laboratory reported pieces of feldspar from Tanzania with blue to white iridescence, a new product called blue moonstone in the industry, which proved to be peristerite. Mindat records peristerite from many localities, for example in Ontario, Canada, and Nordland, Norway.

Classic moonstone shows adularescence, a misty light that seems to roll across the surface, and often small "centipede" inclusions. The Tanzanian peristerite sold as blue moonstone looks similar but has a slightly higher refractive index (1.53–1.54) and specific gravity (2.62–2.63), is inert under long-wave UV, shows red fluorescence under short-wave UV, and lacks centipedes; instead it shows needle-like inclusions and twin planes like labradorite. Its iridescence comes from exsolution lamellae of almost pure albite and oligoclase. Labradorite with multicolored sheen is a different material, sold as "rainbow moonstone". Telling these feldspars apart needs gemological testing.

GIA's advice for moonstone: at Mohs 6 to 6.5 with poor toughness, it can be scratched, chipped or cleaved if knocked against a hard surface, so it suits pendants, earrings and brooches. Clean with warm soapy water; avoid ultrasonic and steam cleaners.

The name moonstone goes back to a German name used by Werner in the late 18th century for a lustrous feldspar. In 2005 the GAAJ laboratory noted that a gem called "blue moonstone" had recently appeared on the market and showed it to be peristerite rather than classic orthoclase moonstone; peristerite is named from the Greek word for pigeon, because its sheen recalls the colors on a pigeon's neck.