Skull
Skulls were one of the types of gems found in Diablo 2. It's not known if they will return as a gem type in Diablo III -- none have yet been seen in the game, while most of the other gem types have already been spotted.
Most gem stats are changed around in Diablo III, so even if Skulls do return, it's unlikely that they will grant health/mana regeneration in helms/armor, or leech in weapons, since those mods are very rare and powerful in Diablo III.