Diamonds are graded on their Cut, Colour, Clarity & Carat Weight.
Diamonds come in many different cuts and sizes. The modern cut for round diamonds is the brilliant cut which is designed to maximise the light reflected off the facets giving exceptional brilliance.
D ~ Absolutely Colourless: Extremely rare diamond with no trace of colour.
E-F ~ Colourless: Appears colourless. Indistinguishable from D grade to the naked eye.
G-H ~ Near Colourless: Very faint tint but appears colourless to the naked eye.
I-J ~ Near Colourless: Faint tint only apparent when compared with higher quality stones.
FL-IF ~ Flawless/Internally Flawless. No inclusions visible even under 10x magnification
VVS1-VVS2 ~ Very very slightly included. Inclusions only just detectable under 10x magnification
VS1-VS2 ~ Very slightly included. Some inclusions, invisible to the naked eye
SI1-SI2 ~ Slightly included. Inclusions may be visible to the naked eye.
I1-I3 ~ Included. Inclusions are visible to the naked eye (sometimes noted as P1-3)
The carat weight is basically the weight of the diamond ~ the higher the carat, the rarer the diamond. Weight gives you a rough idea of size but it isn’t exactly equivalent as the quality of the cut can also make a difference.