A Diamond is a crystal made up of 99.95% pure carbon atoms arranged in an isometric, or cubic, crystal arrangement consisting of tetrahedrally bonded carbon atoms. It is this unique arrangement of the carbon atoms that makes diamond look and act differently from other pure carbon minerals such as graphite.
Actually, a diamond is the hardest known naturally occurring material, scoring 10 on the old Mohs scale of mineral hardness. One major misconception however, is that a loose diamond cannot fracture or break. The reality is that unlike a diamond’s hardness, which only denotes resistance to scratching, toughness relates to its ability to resist breakage from falls or impacts. Because a diamond has certain specific cleavage plains, it is indeed possible for a diamond to chip or fracture if hit with just the right impact in certain spots. It makes no difference if the diamond is loose or has already been set in a beautiful engagement ring. You must always be vigilant in protecting the integrity of your diamond ring.
See more: http://www.exceldiamonds.com/education/diamonds/what-is-a-diamond.php