Steel Release

The is an advanced nature kekkei genkai that can be used to make one's body virtually indestructible by turning it into black steel. The elements combined to create this nature are unknown.

To date, this form of nature manipulation has only been used by Hiruko, who obtained it using his Chimera Technique to steal kekkei genkai in Naruto Shippūden 3: Inheritors of the Will of Fire from an unknown shinobi.

Speculation
The basis for Steel Release may be due to imbuing chakra into the iron and carbon in one's blood, transmuting both elements into steel.