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 a 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 to not go below G because any point below that then you do start to get 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.