Prismatic

Prismatic is a jewelcrafting term referring to the color (sometimes stated as "type") of gem or socket.

When referring to a gem, a prismatic gem matches any color socket. When referring to a socket, a prismatic socket matches any color gem.

Notes

• Prismatic gems and sockets are not interchangeable with meta gems or meta sockets; you cannot put a prismatic gem in a meta socket or a meta gem in a prismatic socket.
• A prismatic gem in your gear counts towards all meta gem color requirements. One  [Nightmare Tear], for example, is enough to activate your  [Relentless Earthsiege Diamond].
• If you equip three prismatic gems you will always have an activated meta gem.
• Prismatic gems can be used to activate set bonuses you would not normally have.

Patch changes

• Patch 7.0.3 (2016-07-19): All existing non-meta gem sockets have been replaced with prismatic sockets.
  
• Patch 3.2.0 (2009-08-04): "In the next major content patch we will be removing the prismatic quality of the jewelcrafter-only  [Dragon's Eye] gems. Like other gems, they will have to match the socket color to receive a socket bonus. When this change occurs, players with qualifying jewelcrafting skill will be provided a yet to be determined amount of Dalaran Jewelcrafter's Tokens as compensation." (Source)