A lump of unrefined carbon, mined from black rocks usually found high on mountains, or in caves. With a metal pick, it is usually dropped in quantities from 1/4 to 1/5 as much as the Obsidian dropped from the rocks. Carbon-committed mining expeditions will end up with a lot of extra Obsidian. With the Obsidian Pickaxe more carbon will be dropped than obsidian.
Can be refined in a forge.
Carbon can be refined into Steel if you place unrefined Iron into a forge. Requires both unrefined Iron (2) and 2 carbon, to make Steel.