Demantoid is named for its diamond-like luster and is a variety of andradite garnet. One of the most distinctive features of demantoid is the presence of characteristic inclusions known as horsetails. The effect is caused by the presence of wispy, hair-like inclusions of byssolite. Demantoids containing well-formed horsetails are highly prized by collectors, and generally command a premium. Demantoid gemstones that feature the Cat's Eye, or Chatoyant optical effect are rare.