Graphite has a sheet like structure where the atoms all lie in a plane and are only weakly bonded to the graphite sheets above and below. Diamond has a framework structure where the carbon atoms are bonded to other carbon atoms in three dimensions as opposed to two in graphite. The carbon-carbon bonds in both minerals are actually quite strong, but it is the application of those bonds that make the difference. Graphite can only be confused with the mineral molybdenite which is metallic bluish silver in color. However, molybdenite is much denser and has a silver blue streak.
Most graphite is produced through the metamorphism of organic material in rocks.
Even coal is occassionally metamorphosed into graphite. Some graphite is found in igneous rocks and also as nodules inside of iron meteorites. Color is black silver. Luster is metallic to dull. Crystal Habits include massive lamellar veins and earthy masses.