Carat

Carat is the "C" that most people already know. This term refers to the carat weight or size of a diamond. The higher the number of carats, the bigger the stone. Alone, carat weight does not determine the value of a diamond. Two, three-carat diamonds can differ widely in price and value when the other "C's" are taken into consideration. 

As well, the shape of the diamond impacts how large a diamond appears. For example, a one-carat marquise or pear-shaped diamond may appear larger than a round or princess shaped diamond. This is primarily due to the pear and marquise shape spreading out the weight.