Meta gem

A Meta gem is a gem that only fits in a meta gem socket. A meta gem socket only accepts a meta gem.

Meta gem requirements
Each meta gem has a set of requirements. These requirements are that the wearer's equipment also have a particular number of other gems. If those requirements are met, the meta gem provides its bonus. If they are not, the meta gem is inactive.

Example:

For this meta gem, if the character's gear has
 * -- at least 2 gems that "fit in #4444ff sockets" (are blue, green, purple or prismatic)
 * -- and 1 gem that "fits in #DC143C sockets" (is red, orange, purple, or prismatic)

this meta gem would be active.

A gem that matches more than one required color (purple or prismatic, for the above example) may satisfy any requirements that it matches. For instance, this meta gem would become active with as little as one blue gem and one purple gem socketed in equipped items.

A prismatic gem on your gear counts for all meta gem color requirements. One Nightmare Tear, for example, is enough to activate your Relentless Earthsiege Diamond.

What equipment has meta gem sockets?
Meta gem slots are sockets that (currently) only appear on particular varieties of headgear. Most often, meta gem slots are found on items intended for "end game usage": level 80 gear, arena gear, dungeon sets, and so on. They also appear on level 70 gear from the Burning Crusade expansion of the same nature.

For a list of such items, see Helmets with meta sockets. Note that this page does not include all headgear that has meta sockets, but does include a good selection of them.

Meta Gem Properties
Meta gems of the same name adjective have the same effects, but may have differing activation requirements.