Titanium

Titanium (Ti), element 22.

A light, strong, non-conductive, non-reactive metal. Perfect for underwater construction—won’t corrode.

Binds with oxygen to form minerals like rutile and anatase, which are likely embedded in bedrock on the sea floor.

Osseointegral metal—can be fused directly and safely to bone. Organisms on the bombardment world Thokcha and more recently encountered variants of the kharaa "onos" xenorhinoid both incorporate titanium into their biology.