Chromium is a rare mineral.
Background[]
It is a mineral found in the caverns in Serenia. It is likely metallic and crystalline.
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.
Chromium is an actual element metal, that can have crystalline forms.
https://en.wikipedia.org/wiki/Chromium
In the computer versions of KQ5 the crystal shatters after reflecting the magic, but in KQ5 on the NES the crystal remains intact and Graham continues to carry it with him.