From Pathfinder Rage of Elements, which isn't enabled.

Ferrofluid Urchin (Moderate)

ITEM 11

CONSUMABLE MAGICAL METAL TALISMAN

Price 225 gp

Usage affixed to medium heavy metal armor

Bulk L

Activate (concentrate)

Prerequisites You have the armor specialization effect of the affixed armor

Trigger You take physical damage.

This spiky glob of magnetic liquid attaches directly onto the metal of your armor. When you activate the globule, it reshapes to deflect the incoming harm. You gain resistance 6 to the triggering damage.

Pathfinder Rage of ElementsOGL