Ironstone

A dense metal specially forged by the Frogkin of Bogroot Swamp.

The Frogkin bury their dead in iron urns packed with sand to protect their ancestors’ spirits, but nothing keeps the swamp out forever. When water leeches through iron, especially over a century or two, bones turn to ironstone, which the Frogkin then use to shape ceremonial weapons. - Backstory Codex (BC-Artificer)