Zircon, ZUR kon, is a mineral composed chiefly of the elements silicon, oxygen, and zirconium. It is a silicate (see Silicate). Zircon contains smaller amounts of other elements, including hafnium, iron, and the rare earths. Some zircons also contain the radioactive elements thorium and uranium. Zircon crystals may be reddish-brown, yellow, green, blue, or colorless. They resemble prisms with pyramid-shaped ends. The crystals occur in alkali basalts, granites, and other igneous rocks (see Igneous rock). Zircon is resistant to weathering, so it is also found in gravel and sand produced by the erosion of igneous rock.