A diamond is a transparent crystal of tetrahedrally arranged carbon atoms. Diamonds are the hardest known natural material and have many industrial applications, including cutting tools, thermal conductors, and opticalwindows. They are also used as abrasives in polishing and grinding. The word “diamond” comes from the Greek adamas, meaning “invincible”.
The carbon atoms in diamond are bonded together by strong covalent bonds arranged in a tetrahedral lattice. This makes diamond very hard—it is the hardest naturally occurring substance—but also brittle; ifHit with a hammer, it will shatter. But because the carbon atoms are held together so tightly, diamonds have a high melting point (about 3550°C) and do not readily react with other chemicals at roomtemperature. They are thus insoluble in all solvents except for hot aqua regia (a mixture of nitric acid and hydrochloric acid).
Diamonds occur naturally in two different crystalline forms: cubic (isometric)and octahedral. About 98% of natural diamonds have the cubic form; these are typically found in kimberlites and lamproites. The octahedral form is much rarer; most octahedral diamonds come fromalluvial deposits or from metamorphosed limestone beds that were originally deposited on the seafloor millions of years ago. Both types of deposit can be found today on every continent exceptAntarctica.