Graphite is a polymorph of the element carbon. diamond is another polymorph. The two share the same chemistry, carbon, but have very different structures and very different properties. Diamond is the hardest mineral known to man, Graphite is one of the softest. Diamond is an excellent electrical insulator, Graphite is a good conductor of electricity. Diamond is the ultimate abrasive, Graphite is a very good lubricant. Diamond is usually transparent, Graphite is opaque.
Diamond crystallizes in the Isometric system and graphite crystallizes in the hexagonal system. Somewhat of a surprise is that at surface temperatures and pressures, Graphite is the stable form of carbon. In fact, all diamonds at or near the surface of the Earth are currently undergoing a transformation into Graphite. This reaction, fortunately, is extremely slow.
It may seem strange that one of the softest minerals (and a very slippery lubricant) is the high-strength component in composites used to build automobiles, aircraft, and of course golf club shafts. It is the weakly bonded sheets that slide by each other to yield the slipperiness or softness. Yet when those sheets are rolled up into fibers, and those fibers twisted into threads, the true strength of the bonds becomes apparent. The threads are molded into shape, and held in place by a binder (such as an epoxy resin). The resulting composites have some of the highest strength-to-weight ratios of any materials (excluding, of course, diamond crystals and carbon nanotubes).
Graphite can only be confused with the mineral molybdenite which is metallic bluish silver in colour. 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 occasionally metamorphosed into graphite. Some graphite is found in igneous rocks and also as nodules inside of iron meteorites.