4 C's of a Diamond + Some

Carat
Probably the most well-known C, carat refers to the weight of the diamond. Like a dollar, there are 100 points in a carat. You may hear a half carat referred to as "50/100ths of a carat" or "50 points." Most fancy-shaped diamonds are elongated in shape and appear larger than a round brilliant of the same carat.

Color

Color refers to the presence of body color in a diamond. Fancy-colored diamonds are very rare and include red, pink, green and blue. The total absence of color is also rare, because most mined diamonds have a slight yellow color and some even have brown or grey.

AGS Color Scale: 0 to 10, 0 being colorless and 10 having body color that is easily seen by an untrained eye

GIA Color Scale: D to Z, D being colorless and Z having body color that is easily seen by an untrained eye

Clarity

Clarity refers to the presence of flaws in a diamond, both internally and on the surface.

AGS Clarity Scale: 0 to 10, 0 being flawless and 10 having a shattered appearance with large, obvious inclusions that effect the brilliance and beauty of the diamond

GIA Clarity Scale: Flawless, IF, VVS1, VVS2, VS1, VS2, SI1, SI2, I1, I2, I3; VVS inclusions are difficult to see under magnification and inclusions become easier to see as you go down the scale; I-clarity stones have inclusions that are easily seen by an untrained eye and effect the brilliance of the diamond

Cut

Sometimes people forget about the cut of a diamond, but it is just as important as color and clarity when considering the beauty of a diamond. The cut is what gives the diamond brilliance and sparkle. Because many people are concerned with carat size, diamond cutters often sacrifice beauty to maximize the carat weight of the stone. After a stone is cut, factors like polish and symmetry are graded through a microscope. Ideal cut diamonds are cut to optimal proportions, which allows the most brilliance, luster and dispersion. Only 5 percent of the round brilliant cut diamonds on the market are cut to this standard.

AGS Cut Scale: 0 to 10, 0 being the best cut with the most brilliance and dispersion and 10 being the worst cut

Shape
Shape is not the same as cut. Shapes include round brilliant, pear, oval, marquise, emerald cut, heart, triangle and other branded shapes. Rounds are the most popular and anything other than round is considered a fancy shape.

Price
Round brilliants often command more money than fancy shapes. In addition, ideal cut diamonds and those with high color and clarity are more valuable than low-grade diamonds.

Hardness/Toughness
Diamond is the world's hardest substance at a 10 on the Mohs scale. It is resistant to scratches, although it isn't impossible to scratch a diamond. And even though it is hard, it is not the toughest substance and it is possible to chip or crack a diamond.

-Information provided by the American Gem Society