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
Prior to patch 7.0.3, each meta gem had a set of requirements. These requirements were that the wearer's equipment also had a particular number of other gems. If those requirements were met, the meta gem provided its bonus. If they were not, the meta gem was inactive.

Example:
 * Shielded Skyflare Diamond

For this meta gem, if the character's gear had
 * -- 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 matched more than one required color (purple or prismatic, for the above example) would satisfy any requirements that it matched. 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 counted for all meta gem color requirements. One Nightmare Tear, for example, was enough to activate your Destructive Skyflare Diamond.

Meta gems added in Mists of Pandaria did not have this requirement.

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 70/80/85/90 gear, arena gear, dungeon sets, and so on.

Meta gem properties
Meta gems of the same name adjective have the same effects, but may have different activation requirements.