One use only. When the wearer suffers its first wound in close combat (before saves, Killing Blow, multiple Wounds, etc), the gem activates. Roll a dice. On a roll of a 1 the wearer suffers the Wound as normal, and an additional Wound with no armour save allowed. On a 2+ the Wound is saved, and is rebounded onto the model that caused it, with no armour save allowed.