Keystone

The Keystone is one of five Bloodstones through which the Gods of Tyria forever shattered magic in the world of Tyria over 1000 years ago. The Keystone is the piece required to assemble the other four.

For more information and background, read the Bloodstone article.

Note: While one of the Mesmer skills is named Keystone Signet, the skill's behavior seem to indicate it is not related to the Keystone of the Bloodstones.