Eye Gems (3.5e Equipment)
From D&D Wiki
These fashionable gems are often used by liches. Placed in the eye sockets of a creature, these gems offer a fancy alternative to a ghastly hollow, empty socket. Gems such as diamonds, star sapphires, or beljuril are often prized. Whatever the type of gem, it is often polished perfectly round, and occasionally engraved with a signature symbol of some sort. Occasionally the back sides of the gem will be faceted (creating a "cat's eye" effect). Over time, many liches eventually end up using their eye gems as soul gems, as well.