Hierro

is the special skin of Arrancar where an Arrancar's spiritual energy condenses and creates a steel hard skin that is strong enough to block a Zanpakutō. Also depending on the Arrancar's power level the stronger the Hierro, Nnoitra Jiruga claims to have the strongest Hierro. It is unknown if his claims are true considering him being the 5th Espada and Ulquiorra, Stark, Halibel, Nel Tu, and Barragan being of higher rank.