When buying an engagement ring, the 4C’s refer to the color, carat, clarity and cut rating of the diamond.
Diamonds are formed deep within the surface of the Earth under intense heat and high temperature. As a result, it is very rare that a perfect diamond occurs in nature and there are often impurities found in the mined rough.
Clarity refers to the visual appearance of the internal characteristics under 10 times magnification. Any flaws and blemishes are collectively known as inclusions. In simpler terms, clarity measures the amount of inclusions found in a diamond. Obviously, the less flaws a diamond has, the rarer and the more expensive it is.
Color refers to the inherent body color the diamond has. Most naturally mined diamond exude warm tones that are yellowish or brownish. Do note that the differences in color ratings are very minute.
When viewed from the top of the diamond, most stones actually appear whiter than they actually are. It takes a trained professional to properly assess the color of a diamond in controlled lab conditions.
Carat is the weight of the diamond and directly affects its value. Larger diamonds are rare and hard to find in nature. This is why a single large sized diamond is often worth much more than several small ones combined together.
If you want to see real life illustrations, you can download a chart here that depicts how big a diamond is according to its carat size.