Diamond

Diamond is one of the allotropes of carbon (along with graphite and Buckminsterfullerines).

Each carbon in diamond is bonded to four other carbons in a tetrahedral structure.

This three-dimensional symmetry (all of the bonds are exactly the same length, strength and at the same angles) makes diamond extremely inflexible. So, if you put a piece of diamond against a piece of glass and drag one past the other, the glass' atoms will be forced out of place rather than the diamonds. This is how diamond scratches glass (or anything else for that matter).

This ability to scratch other things means that diamond is the hardest naturally occurring substance. Be careful - hard in this sense does NOT mean tough. It means difficult to scratch. Diamonds can be chipped and shattered, they just can't be scratched by anything that occurs naturally other than another diamond.