Carat is the term used to describe the weight of any gemstone, including diamonds. Diamond weights are stated in metric carats or "ct" (versus the karat amount of gold contained in an item). Since 1913 the international standard has been 1 carat = 200 milligrams, or 1/5 of a gram. In jewellery pieces with more than one diamond, the carats may be described in terms of total carat weight (TW). This number is the combined total weight of all stones in the piece.

Clarity

Most natural diamonds contain identifying marks known as inclusions, many of which are invisible to the naked eye. Diamonds having fewer and less noticeable marks are rarer and command higher prices. FL Flawless Free from all inclusions or Blemishes at 10x magnification. IF internally flawless no inclusions visible at 10x magnification VVS1 VVS2 insignificant surface blemishes.VS1, VS2 minor inclusions difficult to see face-up at 10x magnification SI1,SI2 noticeable Inclusions easy to see at 10x magnification. I1,I2, I3 obvious inclusions easily visible to unaided eye

Colour

Most diamonds appear colorless but actually have slight tones of yellow or brown. Diamond color is graded on a scale of D (colorless) to Z (heavily tinted), and the closer a diamond is to being colorless, the more valuable it is. Even when a diamond has a visible tint such as K or above, it can still be beautiful if it has good clarity and cut.

Cut

Quite often the cut of a diamond is confused with its shape. Diamonds are cut into various shapes depending upon the original form of the uncut diamond, which is referred to as "rough". Whatever the shape, a well-cut diamond is better able to reflect light.
The Cut refers to the proportions, finish, symmetry, and polish of the diamond. Of the 4C's, the cut is the aspect most directly influenced by man. The other three are dictated by nature.