Maria Doulton: Another important 'C' to consider when buying a diamond is colour, and if it's a white diamond, ironically what you're looking for is total lack of colour. Now, the top grading for a white diamond is D, and then it goes down alphabetically right to Z. The way that they decide on the grading for the colour of a diamond is to look at it upside down - eliminating the sparkle factor - against white paper, and then they check that against a chart of colours, so it's all scientific and it's determined by an expert.
If you're considering buying a single-stone ring, the general recommendation, and that of Graff as well, is not to go below G because any point below that then you do start to get a slightly less clear white colour.
Now, coloured diamonds come in blue, pink, green and, of course, yellow, and I have here a Vivid Yellow diamond. This diamond, because a yellow diamond of this quality is so rare, would be worth many more times an equivalent diamond in white.