CL (Coal?) Small portion
A black material that can be mined with the pickaxe.
According to the material's image name in the game folder from Alpha 35, ORECL-0.TGA, it would seem to be coal, although Cl is Chlorine on the periodic table.
Now one can only speculate about what uses it will have in the future as it is unuseable in Lords of Uberdark Alpha 37.