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London blue topaz is typically produced by exposure to radiation in a nuclear reactor. When topaz is exposed to fast neutrons, the radiation changes the color centers, producing the deep blue color.

Blue topaz is a popular variation of the topaz gemstone. ... Although the color of topaz does not fade easily, exposing the stone for an extended period to high heat or direct sunlight may alter the gem's hue over time.

Topaz's hardness (8 on the Mohs scale) makes it durable and means that it does not scratch easily. To clean your topaz, simply use soapy water and a soft cloth. Be sure to rinse well to remove soapy residue. As with most gemstones, ultrasonic cleaners and steamers are not recommended.