Cut is possibly the most important of the four Cs. For round brilliant diamonds, cut refers to the quality of workmanship (proportion and arrangement of facets), not the shape. The amount of brilliance, sparkle, and fire displayed by a diamond is determined by the quality of its cut.
The scale runs from 'Excellent' to 'Poor.' The majority of diamonds on the market are cut from 'Excellent' to 'Very Good.' Diamonds with a cut grade of 'Poor' will appear lifeless and dull.
The absence of colour in a diamond is measured by colour. This is due to the fact that most "colourless" or "white" diamonds contain trace amounts of yellow, brown, or grey colour. A diamond with less colour is more desirable and valuable.
The D-to-Z colour scale has 23 colour grades, with D indicating that a diamond has no detectable colour and Z indicating that a diamond has "light" colour. Any diamond with a colour grade greater than Z is considered a fancy colour diamond and is graded on a different colour scale.
The price of a stone is affected by colour grade, but differences of one to three colour grades are not easily discernible to untrained eyes. Diamond graders examine diamonds face down in special environments to detect subtle colour variations.
Diamonds of the D, E, and F grades are extremely rare and valuable. G and H diamonds are generally thought to be good buys.