The weight - Carat, the rule: 1 ct = 0.20g = 200 mg = 1/5 g
For stones less than 1 ct, divide into 100 points. Ex: 0.45ct = 45 pts
Diamonds that are less than 20 pts are called "melee".
You have to round to the 3rd digit after the nine. Ex: 0.479 ct = 0.48 ct / 0.477 ct = 0.47 ct.
Due to the rarity of big stones in nature, one diamond of 2 ct has a value well above two diamonds of 1 ct.
Diamonds are classified in categories, according to their weight. Ex: the category of 1.50 ct to 1.99 ct, the category of 2 ct to 2.99 ct, etc. Therefore, despite their almost equivalent weights, a 2.01 ct diamond will cost substantially more than a diamond of 1.98 ct.
Round brilliant diamonds are significantly more expensive at equivalent weight and require more work for the diamond cutter than other sizes, this being due to the complexity of their symmetry.