Carat refers to a diamond's size, which is measured in carat weight.
1 carat is 1/5 of a gram.

Each carat is equal to 100 points, so that .50 carat diamond is the same as a
50-point diamond or a ½ carat diamond.

In nature, larger diamonds are not often found. However when discovered,
a larger diamond is treasured for its beauty, and rarity.
A larger diamond will accentuate a diamond's brilliance,
thus increasing the desire to own one.

While larger diamonds are certainly treasured for their beauty and rarity,
diamonds of equal size may vary considerably in brilliance and value
depending on their Cut, Color and Clarity. It is best to use the 4-C's as a guide
to balance Cut, Color, Clarity, and Carat weight so as not to underestimate
the significance of each of the 4-C's as they affect a diamond's beauty. 

DIAMONDS
CUT - COLOR - CLARITY - CARAT