Anthracite

All forms of coal are considered sedimentary rocks except for Anthracite which has just barely metamorphosed from Bituminous coal. Anthracite has a metallic sheen but while it looks pretty it is too soft (hardness 2.5) and light-weight to easily work. The example above looks round but it is actually a little oblong and indented compared to the symmetry achievable in harder stones. Anthracite is at least 86% carbon and highly flammable. In the U.S. there are more than 100 underground coal fires burning currently. The Lewis and Clark Expedition reported fires burning in coal seams in Wyoming and Montana and in Pennsylvania there has been a continuous underground Anthracite fire burning since 1962 causing nearby Centralia to become a ghost town.