All piercing damage is physical damage. Most piercing-type damage is caused by daggers and arrows fired from bows. The ranger skills Barbed Trap and Spike Trap and the pets Black Widows and Dune Lizards cause piercing damage as well.
Necromancers Bonelace Armor has +15 Armor against Piercing Damage. Most bows start out with piercing damage, but can be modified with strings for different types of damage, i.e. an icy bowstring for cold damage, etc.
