English: A large block of topaz and a small sample of irradiated topaz (in bright blue) on display in the Hall of Gems and Minerals at the Smithsonian Museum of Natural History in Washington, D.C.

Topaz is a mineral of composed of two aluminum atoms, a silicon atom, and four oxygen atoms, with two fluorine hydroxide atoms chaining off them.

Pure topaz is colorless and transparent. But when there are other elements present, they impart color and tend to make the gemstone translucent. Topaz can come in a range of colors: dark red, yellow, pale gray, reddish-orange, blue brown, white, pale green, blue, gold, pink, or reddish-yellow.

Many ancient texts (like the Bible) mention topaz. That's because a yellowish clear crystal was mined on the island Τοpáziοn (now St. John's Island in the Red Sea). People called it topaz, but in fact it was really yellowish chrysolite. In the Middle Ages, topaz meant "yellow crystal" and did not mean real topaz. What modern people call topaz was not discovered until probably 1400 or 1500 AD.

Exposing topaz to gamma radiation can produce a brownish or brownish-green color. If the stone is then heated, the heat breaks down the molecules that reflect yellow light, thus leaving only a light or medium-light blue color. Many topaz stones are treated with gamma rays to see how much blue is in them.

Topaz that is exposed to a high-energy electron beam both generates radiation and high heat. The stone is then cooled with water, and then heated again to remove the yellow-causing impurities. Stones treated this way are of a deeper blue. They also are radioactive for a couple of weeks, and must be allowed to discharge their radiation first!

Topaz that is exposed to fast neutrons, like those generated in a nuclear reactor, produced a dark deep blue. Heat treatment again occurs, to remove any smokiness. These stones are highly radioactive for many months or even years.

A combination of electron beam, nuclear reactor, and heating produce a vibrant, sparking electric blue. Again, they are highly radioactive for some time.