Diamond is a polymorph of the element carbon. Graphite s another polymorph. The two share the same chemistry, carbon, but have very different structures and properties. Diamond is hard, Graphite is soft (Which is also present in the "lead" of a pencil). Diamond is a good conductor of electricity, current can pass easily through it. Diamond is the ultimate abrasive, Graphite is a very good lubricant. Diamond is 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