Zircon is an important gemstone of many colors, and is an historical gemstone used for thousands of years. It is sometimes looked upon as a cheap Diamond simulant, but in actuality it can be a valuable gem.
Its color diversity is caused by traces of certain impurities, some of which are radioactive. Zircon gemstones containing radioactive element traces undergo a process called metamiction, in which their inner crystal structure becomes destroyed. These radioactive forms of Zircon must be heated to stabilize them for use as gems. In fact, many of the gem forms of zircon (even those that are not radioactive) are heat treated to enhance color and increase transparency.
The radioactivity in Zircon gemstones is very minimal and is generally believed to be safe, without posing any health hazards.