Carbon Planet

A Carbon Planet is a terrestrial planet composed largely of carbon, rather than iron and/or silicates, that could potentially form in oxygen-poor, carbon-rich star systems (such as around brown dwarfs, white dwarfs, neutron stars or closer to the galactic core). No water could exist on a carbon planet, as it would react with the carbon.

A carbon planet would probably be structured as follows: Volcanism on a carbon planet could push diamonds from the subsurface layer to the surface, resulting in mountains of diamonds and silicon carbides. If surface temperature is less than 77℃, a weather cycle is possible. Temperatures below 350 K would allow long-chain hydrocarbons to fall down as rain. On carbon planets, rivers and lakes of oils could conceivably exist.
 * A steel or iron core
 * A layer of molten silicon carbide and titanium carbide
 * A layer of graphite or, if pressures are great enough, diamond
 * A surface layer, where liquid or frozen hydrocarbons such as methane and tar could exist
 * An atmosphere of carbon monoxide and/or carbon dioxide, with carbon smog

Worldbuilding in Practice
"Abrota, a small carbon planet orbiting at 1.04 AU, is the only planet to orbit the brown dwarf Nisos. Abrota is infamous for its numerous mines and industrial complexes, owned by the Extrasolar Investment Corporation, whose workers are almost entirely indentured servants and the exploited poor, who earn next to nothing. Worker deaths are numbered in the hundreds of thousands, often due to cheap, faulty environmental suits. Despite this, Abrota's horrors are relatively unknown: swept under the proverbial carpet by governments who benefit from Abrota's exploitation."