Zircon always occurs in crystals that are pointed at both ends, although the middle may be stretched into long prisms. Most often brown, zircon also can be blue, green, red, or colorless. Gem zircons are usually turned blue by heating brown or clear stones.
Zircon has a very high melting point, is fairly hard (Mohs hardness 6.5–7.5), and is resistant to weathering. As a result, zircon grains can remain unchanged after being eroded from their mother granites, incorporated into sedimentary rocks, and even metamorphosed. That makes zircon valuable as a mineral fossil. At the same time, zircon contains traces of uranium suitable for age dating by the uranium-lead method. Zircon is the gold standard for dating ancient granites, which can be hard to date with other methods.
Learn more about zircon and other zirconium minerals.