precious stone

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of several gems, including the diamond, emerald, ruby, and sapphire, that have high economic value because of their rarity or appearance.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any gem, such as a diamond or ruby, that is expensive because of its rarity or desirability; especially one set into a piece of jewelry.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a precious or semiprecious stone incorporated into a piece of jewelry