You just forfeit the chance to get the socket bonus for that item. I would assume it is not possible to socket a blue gem in a red slot It is, in fact. There are eight types of Gem: Red, Blue, Yellow, Orange, Purple. Guy has 6 slots and just 2 coal in them. even 6 bottom tier amethysts(I think that's the purple gem) would have changed the fight I think. Likewise, Meta gems are the only type of gem that will fit a Meta socket. meta-gem socketing:P ; Mastropiero - Warrior 80 ~ Zuriel - Paladin 80 ; Converter - Druid 80 ~ Megahertz - Rogue 80 ; Thaz - Shaman 80 ~ Bobbit - Death Knight . Only fits in a meta gem slot.