"The oldest known rock in the world is generally believed to be a zircon from Canada estimated at nearly 4 billion years old." -- Gems: A Lively Guide for the Casual Collector by Daniel J. Dennis.

Zircon is a very interesting mineral -- non-gem zircon rock is mined to extract the zirconium metal used in various alloys. Pure zircon crystals are colorless and used to be sold as a diamond substitute, although they are doubly refracting and hence even more sparkly than real diamond (they're also much more brittle). Colorless zircon can also be made from the more common brown stones by heat treatment; blue and golden-yellow stones are also made this way. The most common non-treated zircons are honey-brown and called "hyacinth."

A few varieties (called "low zircon") are actually very slightly radioactive -- not enough to hurt their wearer, but enough that their crystal structure breaks down and the stone loses its luster. The jewelry trade avoids these as much as possible.