Zircon is a zirconium silicate mineral (ZrSiO4) that is found in igneous, metamorphic and sedimentary rocks. It often contains unstable elements such as hafnium, uranium and thorium. The presence of uranium and thorium sometimes makes crystals of zircon useful for determining the age of the rocks in which they crystallized, such as granite. Zircon is harder, more resistant to weathering and has a higher specific gravity than most other minerals. This enables it to be concentrated in placer deposits where it is sometimes recovered for use as a gemstone. Colorless zircon was once a common substitute for diamond.