London Blue Topaz

"London" is not a source locality. London Blue topaz is produced by treating colorless natural topaz, which is abundant and occurs as large, clean crystals; naturally strong blue topaz is extremely rare. GIA notes that treaters use a combination of radiation and heat to produce blue topaz, and the 2011 Gems & Gemology study irradiated near-colorless pegmatite topaz from Guangdong, China, in a nuclear reactor to obtain this color.

London Blue is a color description within blue topaz, judged by eye: it is darker (moderate to dark tone) than the brighter, lighter Swiss Blue. The stone itself tests as topaz (refractive index about 1.61–1.64, specific gravity 3.4–3.6, hardness 8). Neutron irradiation can make trace elements in topaz radioactive; in the 2011 Gems & Gemology study most samples were safe 95 days after irradiation, but some required several years of quarantine. Regulations require treated topaz to be held until its residual radioactivity falls below the exemption level (74 Bq/g cited in that study) before release.

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 treated blue color is permanent and does not fade with exposure to light.

According to GIA, blue topaz, which includes the London Blue grade, is the gem of the fourth wedding anniversary, and topaz is one of the US birthstones for November.