The strength of the individual C-C bonds and their arrangement in space give rise to the unusual properties of diamond.In some ways, the properties of graphite are like those of diamond. Whereas diamond is the hardest substance known, graphite is one of the softest.
As a result, graphite is soft enough to be used in pencils and as a lubricant in motor oil. (This is fortunate because many people chew pencils and lead compounds are toxic.) Lead pencils contain graphite, or "black lead" as it was once known, which is mixed with clay (20% to 60% by weight) and then baked to form a ceramic rod.At very high temperatures and pressures, diamond becomes more stable than graphite.In 1955 General Electric developed a process to make industrial-grade diamonds by treating graphite with a metal catalyst at temperatures of 2000 to 3000 K and pressures above 125,000 atm.The properties of diamond are a logical consequence of its structure.Carbon, with four valence electrons, forms covalent bonds to four neighboring carbon atoms arranged toward the corners of a tetrahedron, as shown in the figure below.Increasing the percentage of clay makes the pencil harder, so that less graphite is deposited on the paper.