Colorless when pure, the zirconium silicate takes on various shades due to impurities. The original diamond substitute, colorless stones, if well cut, can be convincing, but are easily distinguished from diamond by their double refraction and the tendency to wear along facet edges. Brownish stones are often heated either with or without oxygen present to achieve shades of blue and golden yellow. Some crystals contain radioactive thorium and uranium. Although use in rings should be limited to protective settings or occasional wear jewelry, in general zircon is a magnificent jewelry stone.
Collectors appreciate the many color forms but especially seek out reds and greens.