In graphite, carbon atoms are arranged in sheets that easily slide past each other, making them ideal as lubricants and of course, pencil leads. In order to achieve such a compact and strongly-held network of carbon atoms, it is believed that diamonds must have crystallized deep under the Earth's surface. Such pressures and temperatures reproduced in laboratories have successfully yielded synthetic diamonds. They are also found in sedimentary rock where gravel deposits and organic material have been compressed into rock. Diamonds found in kimberlite are thought to be very old, perhaps as much as three billion years old.
Diamonds are crystals, crystals being the ultimate form of symmetry in nature.
The shape of the crystal reflects the internal orderly arrangement of atoms within it. In diamonds, atoms of carbon are held tightly by covalent bonding, where two neighboring atoms share an electron, endowing the diamond crystal with great strength. But despite its hardness, diamonds can be cut. This is accomplished by cutting the gem along planes parallel to the faces of the crystal where the tight bonds between carbon atoms are a little weaker. Found in their natural form, diamonds can appear quite unimpressive.