A Steel Spine is an enchanting crystal that can be used on an enchanted forge on any enchantable item. This gem will modify any weapon to increase the Physical Damage of a weapon on a successful hit, as well as modify any armor to damage an attacker that does a melee blow on the player.
This item was introduced in Geneforge 3 and has stayed in the game ever since. A shaper with all enchantable equipment with Steel Spines can end up doing more damage back on their attacker than the attacker does to them. This, of course, is rare due to the Steel Spine's unlikeliness to be found.