The weight of a diamond is expressed in carats. One carat equals 0.2 grams or 100 points, which indicates the size. HRD Antwerp has extremely accurate scales that measure up to 1/100,000 of a gram. This accurate weight is very useful for the identification of the diamond.
With clarity, we describe a diamond's purity. As they are a product of nature, all natural diamonds have traces of their growth history. In gem quality diamonds these are minor internal characteristics. The clarity scale reflects the size, number and location of the characteristics when examined with a 10 x loupe. Diamonds that have no inclusions visible to the naked eye are of excellent quality. The very best - and rarest - clarity is called 'loupe clean'. This means that upon examining the diamond with a loupe, no inclusions can be found.
Most diamonds range in colour from colourless to slightly yellow. The colour grade is determined by comparison with a series of master stones, the best grade being colourless - a perfect diamond has no colour at all (D Colour: Exceptional White +). Diamonds can also occur in all ranges of colours, from brownish to striking yellow, pink, purple, red, blue and green. These are called 'fancy colours'. Some diamonds react to UV-light (black light) with a grading scale for fluorescence, which ranges from 'nil', 'slight' or 'medium' to 'strong'.
A diamond's cut is essential to its beauty. The cut is divided into 3 grades: proportion, polish and symmetry. Proportions determine the brilliance and 'fire' of a diamond, symmetry describes the variation of different parameters that define the proportions and polish describes the finish of the facets. Each grade is evaluated according to four parameters: Excellent, Very Good, Good and Fair. If the diamond is not well cut, it will not interact with light as it should. A perfect cut equals more sparkle, brilliance and fire.
A common misunderstanding is that the term 'diamond cut' also refers to the diamond's shape. Cut is what determines how well-cut your diamond is, whereas shape is the form into which it was cut.
Diamonds can be cut into various shapes, the best-known of which is probably the round brilliant.