Diamond weight is measured with more precision using metric carats, abbreviated "cts". One metric carat is 1/5 of a gram, just over 0.007 of an ounce. Whereas 1oz. contains almost 142 carats.
Pricing diamonds per carat is how most wholesalers quote diamond prices. For example, one might say he has a 1.25-carat diamond for $3000.00 per carat. To find the total cost, multiply the per-carat price by the weight. In this case, $3000.00 X 1.25ct = $3750.00.
The degree of rarity in a diamond is reflected in its per-carat price.