Keystone

Keystones are objects found when digging for archaeology artifact fragments using the archaeology profession. The keystones are specific to racial artifacts (i.e. there are dwarven keystones, night elven keystones, and so on) and add 12 fragments to some archaeology projects, though not every project will allow the player to use a keystone.

When a player can use a keystone to complete a project, a hexagonal socket will appear at the bottom left of the project window. If the player has a keystone in his or her inventory, the player may then left-click the socket and the keystone will be placed in it (though it will not be consumed until the player solves the artifact). The keystones may be used exactly like 12 fragments, meaning that if a keystone gives more fragments than necessary to complete an artifact, the extra fragments will roll over to the next artifact project. If a player changes their mind about using a keystone before solving the project, they can left-click the socket again to remove the keystone.

Common artifacts that permit keystones only allow the use of one or two. Rare-quality artifact projects may permit the use of up to three keystones. There are no keystones available for use with Fossil projects.

Keystone types
There are several known kinds of keystones, each matching a race.

Keystone dungeon buffs
Some keystones can be used near the entrance of Cataclysm dungeons to grant a group-wide buff for the duration of the run.

Trivia

 * The Keystone is a type of artifact in StarCraft II.