Diamantine. I've seen small chunks of it before, but never a complete lithon. We could make our fortune with one of these, lass—if we could get it out of here. This stuff is almost as hard as diamond and it retains sunlight.
– Marc, Dark of the Moon, "Chapter 5: Under Green Leaves"
Diamantine is a rock used by the ancient peoples of Rathillien for various artifacts, such as the large imus in the Anarchies and the Heart of the Woods. It "retains sunlight" and thus glows in the dark; it is almost as hard as diamonds (hence the name). Marc tells Jame that it is rare and expensive, and that a single lithon like they find in the Anarchies could make their fortune if they could get it out of there.