Definitions of gem:

• noun:   art highly prized for its beauty or perfection
• noun:   a crystalline rock that can be cut and polished for jewelry

  Example: "He had the gem set in a ring for his wife"

• noun:   a precious or semiprecious stone incorporated into a piece of jewelry
• noun:   a sweet quick bread baked in a cup-shaped pan
• noun:   a person who is a brilliant and precious as a piece of jewelry