Adamantine Skeleton (3.5e Feat)
From D&D Wiki
Adamantine Skeleton [Necromantic]
Undead you create have their skeleton structure reinforced by Adamantine
Benefit: When you create an undead creature you may choose to add 100 gp worth of Adamantine and an emerald worth at least 50 gp to the material components of the spell. If you do, the undead creature gains 8 hitpoints per Hit Die. A skeleton created with this feat also gains +4 Strength, +8 natural armor, DR 8/-, gains +4 to hit, and deals +4 damage with its Natural Attacks which bypass hardness less than 20. An undead created in this way counts as four Hit Die higher for the purposes of creating it and controlling it.
Special: This feat may only be applied to undead that have an intact skeleton (skeletons, zombies, ghouls, but not ghosts, for example). You may not apply this feat to an undead in conjunction with any other feat that reinforces its skeleton.