Blue Topaz

In nature topaz is most commonly colorless, and naturally strong blue gems are extremely rare (Geology.com illustrates a natural blue crystal from Zimbabwe). Commercial blue topaz is made from abundant colorless topaz, which is available as large, clean crystals. Topaz in general is mined in Brazil (notably Minas Gerais), Pakistan, Namibia, Nigeria, Madagascar, Mexico, Myanmar, Sri Lanka, the United States and Russia.

Blue topaz is identified as topaz by its refractive index (about 1.61–1.64), specific gravity (3.4–3.6) and hardness of 8. Whether a blue color is natural or treated usually cannot be proven: GIA's Gems & Gemology reported in 1985 that no routine gemological test could definitively tell natural from treated blue topaz, and GIA notes that the blue color is almost always caused by treatment. Trade grades describe the result: Swiss Blue is bright with a light tone; London Blue is darker. Neutron-irradiated stones can be temporarily radioactive and must be held until they decay to a safe level before release; national regulations control this. Topaz with a rainbow-colored surface ("Mystic Topaz") is colorless topaz coated with a thin artificial film, a different treatment.

Topaz is 8 on the Mohs scale but has poor toughness and one direction of perfect cleavage, so it can chip or split if knocked or handled roughly. GIA and Geology.com advise against steam and ultrasonic cleaning; high heat or sudden temperature changes can cause internal breaks, and liquid inclusions can make a stone fracture when heated. Clean with warm, soapy water. Topaz may be affected slightly by some chemicals. The blue produced by irradiation and heating is stable to light and does not fade in normal wear; it is lost only on strong heating (about 500 °C).

The production of blue topaz from colorless topaz by irradiation was first reported by F. H. Pough in 1957 and rediscovered by Kurt Nassau, who reported it in 1974 and 1975. Since the 1970s treatment has turned blue topaz into a mass-market gem: it first sold for $20 to $40 per carat, but oversupply drove wholesale prices down to a few dollars or less per carat. Before about 1970, most topaz in affordable jewelry was yellow to brown.

According to GIA, blue topaz is the gem of the fourth wedding anniversary, and topaz is one of the US birthstones for November. Beliefs recorded for topaz in general include the ancient Greek belief that it gave strength, the Renaissance European belief that it could break magic spells and dispel anger, and a long-held belief in India that topaz worn above the heart assures long life, beauty and intelligence.