Carat is the weight of the diamond and the metric that denotes its size. Although size is an important factor in determining the value of a diamond (the greater the carat weight, the rarer and more valuable the diamond becomes), the cut, colour, and clarity are equally important.
Colour is the hue of the diamond or the lack thereof. While white or colourless stones are traditionally used in diamond engagement rings, diamonds are found in a wide spectrum of colours. Engagement ring diamonds are graded from D to Z, with D being the prized colourless diamond. The hue of D, E and F coloured diamonds is difficult to differentiate to the untrained eye, but the amount of colour becomes more apparent as the alphabet progresses. Z graded diamonds are a pale yellow or brown colour, and anything falling outside of this range is considered a fancy coloured diamond. The colour of the metal in a mounting can either mask or enhance the diamond colour. Yellow gold makes slightly yellow or brown diamonds appear more colourless. If a diamond is mounted in white gold or platinum, the colour becomes more apparent.
Clarity is a judgment of the internal presence of inclusions (birthmarks) and the external visibility of blemishes. These make each diamond individual and unique. The regularity, size, nature, and position of these characteristics is considered and graded on a scale of F (Flawless/no inclusions) to I3 (inclusions apparent to the naked eye). These are graded according to GIA, the worldwide standard for grading gems.