Chromium

Chromium is a rare mineral (likely crystalline and metallic) material found in the caverns in Serenia.

Chromium is resistant to magic.

Graham once mined a piece during his adventures in Serenia (KQGS). He later crafted it into a lock that was resistant to magic and required multiple keys.

Behind the scenes
It's properties of being mined and having been resistant to magic is likely a reference to the Crystal mined from the Crystal Caverns in KQ5 which was capable of reflecting magic back at its source.