The weight - Carat, the rule: 1 ct = 0.20g = 200 mg = 1/5 g

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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".

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You have to round to the 3rd digit after the nine. Ex: 0.479 ct = 0.48 ct / 0.477 ct = 0.47 ct.

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Due to the rarity of big stones in nature, one diamond of 2 ct has a value well above two diamonds of 1 ct.

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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.

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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.