The cut of a diamond refers to the quality of the diamond cut (including its symmetry, proportioning and the polish of its facets). It does not, as is often misunderstood, refer to its geometrical shape (round brilliant, princess, emerald, cushion, etc.). The cut contributes more to the beauty of a diamond than any other factor. A high quality cut really enhances a diamond’s brilliance (brightness), sparkle or scintillation (flashes of light and dark) and fire (flashes of colour).
The GIA assigns the following cut grades:
Nearly all diamonds contain small imperfections known as inclusions if they are on the inside of the diamond and blemishes if they are on the surface of the diamond. These imperfections tend to alter the path of light through the diamond and so, if a diamond has many inclusions or blemishes, it is likely to appear less brilliant. When grading the clarity of a diamond, the grader will view the diamond both with the naked eye and through a loupe (a 10x magnifying glass).
The GIA has develop the following standard definitions for levels of clarity, which are now widely used:
Internally flawless (IF)
Very, Very Slightly Included (VVS1, VVS2)
Very Slightly Included (VS1, VS2)
Slightly Included (SI1, SI2)
Included (I1, I2, I3)
There are many, many colours and hues of diamonds, including blue, green, yellow, orange, red, pink, purple, brown and even black. Diamonds that are chosen for engagement rings, however, tend to be white diamonds. White diamonds almost always contain impurities that impart a yellowish tint to the diamond. The most sought after diamonds are those with a very faint yellow tint, imperceptable to the naked eye.
The GIA developed the following grading system for this yellow colour in white diamonds:
Colourless (D, E, F)
Near Colourless (G, H, I, J)
Faint Colour (K, L, M)
Very Light Colour (M – R)
Light Colour (S – Z)
The weight of diamonds is measured in carats or sometimes points. One carat equals 200 milligrams (or 100 points). As carat weight increases, diamonds become more and more rare. For this reason, the price of diamonds rises disproportionately to weight increases. In additions, there tends to be sharp jumps in cost around the milestone carat weights.