This post consists of two parts. The first part is about graphite and the second part is about diamonds.
Graphite and diamond are native element (ones that consists of a single element). Like graphite, diamonds consist of pure carbon. These two natural forms of the element carbon (C) have the greatest contrast in structure and properties to be found in any pair of polymorphous substances.
Graphite has its atoms arranged in a hexagonal structure, which forms sheets that are poorly connected, thereby allowing the sheets to easily slide over one another if subjected to a small amount of force. Thus, graphite is slippery and greasy. That is why it is used in pencils (as in "lead" pencils) and also is used as a lubricant. Graphite is, furthermore, light, extremely soft, inert, and highly resistant to chemical changes. Graphite is a good conductor or heat and electricity and is used for making solar panels, batteries, and electrodes.
Specimen of graphite (3.2 cm from bottom to top of image).
Graphite forms predominantly in rocks affected by regional or contact metamorphism, and it is found in crystalline limestone, metamorphosed coal beds, schist, gneiss, quartzite. Locally, it can form in veins (pegmatites). Under high pressure and temperature, graphite converts to diamond. Graphite has been found also as minute crystals in some meteorites.