Zircon

Zircon is volcanic in origin, found in igneous rock formations. It is a brownish crystal that turns blue when heated sufficiently and can be cut into faceted gemstones. In the Realms, zircons were sometimes substituted for more valuable gems to fool the uninitiated, but anyone with a passing knowledge of gemology (a jeweler, a dwarf, or even a seasoned adventurer) could notice the difference. A typical specimen had a base value of 50 gp.