The skulls of some demonic creatures, inscribed with eldritch symbols, are said to work much like gems. Skulls were introduced as socketable items in Diablo II, did not appear in Diablo III and Immortal, and returned for Diablo IV.
Weapons: Adds Mana and Life Steal to attack Shields: Adds attacker takes damage Helms and Body Armor: Adds Mana and Life regeneration