Diamond weights are measured in ‘carats.' One carat equals 1/5th of a gram. In other words, there are 5 carats in 1 gram.
You also may have heard the term ‘points' when discussing diamond sizes. One carat is divided into 100 points so each point is 1/100th of a carat. For example, a "10-point" diamond weighs 1/10th of a carat, and a 50-point stone weighs one-half carat.
The top color for a diamond is actually a ‘colorless diamond'- one without any color in it at all. A ‘D' color diamond is a perfectly colorless stone and is the most appealing for many people. The color grade then goes down the alphabet with each letter denoting a lower grade of color.
Although a ‘Z' color diamond is the lowest and considered least appealing, once diamonds are ‘Z'+ meaning they have a great saturation of color in them, they are then considered ‘fancy color diamonds' and are extremely rare (and expensive).
Clarity refers to the presence of internal and external characteristics in a diamond.
1 is clearer (has fewer inclusions) than 2
example: VVS1: very very slightly included 1
The cut refers to the angles and proportions your diamond contains. A well-cut diamond will internally reflect light from one mirror-like facet to another and disperse and reflect it through the top of the stone resulting in the sparkle, brilliance and fire flashing out of your stone.
Diamonds that are cut too deep or too shallow lose or leak light through the side or bottom and so reflect less sparkle, brilliance and fire.
The cut also refers to the finishing qualities, being symetry and polish, and the shape of a diamond (round, oval, pear, marquise, heart etc).