Zircon is a remarkable mineral, if only for its almost ubiquitous presence in the crust of Earth. It occurs in igneous rocks (as primary crystallization products), in metamorphic rocks and in sedimentary rocks (as detrital grains). Large zircon crystals are seldom abundant. Their average size, e.g. in granite rocks, is about 0.1–0.3 mm, but they can also grow to sizes of several centimeters (a few inches), especially in pegmatites.
Owing to their uranium and thorium content, some zircons may undergo metamictization. The processes, related to internal radiation damage, partially disrupt the crystal structure and partly explain the highly-variable properties of zircon. As zircon becomes more and more modified by internal radiation damage, the density decreases, the crystal structure is compromised, and the color changes.
