Things you need to know about diamond carat weight:
1) Carat weight is the biggest factor affecting the price of the diamond!
2) Diamonds are measured in carat weight. One carat is 100 points. Ex: A 75 point diamond = 3/4 carat or 0.75ct. Carat is expressed as "carat" or "ct". Do not confuse carat with "Karat" or "kt", which refers to the purity of gold jewelry.
3) On the wholesale level diamond prices are quoted in price per carat. Ex: if a diamond costs $5000 per ct and weighs 1.25 carats, the actual cost of the diamond is $7500. If the diamond costs $5000 and weighs 0.75ct, it will actually cost you $3750. This is good to know if you are comparing two diamonds. One weighs 1.02ct and costs $10,350 (10,350 /1.02 = $10,147.06 per ct) and another that weighs 1.06ct and costs 10,500 (10,500/1.06 = $9,905.66). In this case The more expensive diamond is actually $241.40 cheaper per carat and is actually a better bargain assuming all other factors are the same (which is usually not the case since there are over a trillion different combinations that could affect the price of a diamond of the same size color and clarity).
4) Although a diamonds weight is usually stated to the closest hundredth (ex: 1.01ct), on the wholesale level, the price is usually calculated to the nearest thousandth (ex: 1.019ct). This means that if you are looking at two 1.00ct diamonds that both cost $10,000 but one actually weighs 1.009ct while the other one actually weighs 1.000ct, the price per carat is actually $90.00 cheaper on the 1.009ct.
Of all the possible combinations of seemingly identical diamonds, if you only factored in the last two weight related items described here, you could easily have a $500 or even $1000 difference in the price.
5) You should know that diamond prices are not linear. If you are price shopping a 1/2ct diamond for $2000 and then you change your mind and decide to go for a 1ct diamond, don't expect to pay $4000 for a 1.00ct diamond of the same quality. It will be much more. This is because the larger the diamond, the more rare and valuable it is.
Diamonds have different weight categories listed below. Each category represents a different price per carat for the same quality. Whenever preforming a search online, be sure to narrow your search criteria down to just one specific range. Search diamond prices.
1/4 ct: 0.20 to 0.29
1/3 ct: 0.30 to 0.39
40 pointer: 0.40 0to 0.49
1/2ct: 0.50 to 0.69
3/4ct: 0.70 to 0.79
80 pointer: 0.80 to 0.89
90 pointer: 0.90 to 0.99
1 ct: 1.00 to 1.19
1 1/4ct: 1.20 to 1.49
1 1/2ct: 1.50 to 1.69
1 3/4ct: 1.70 to 1.99
2 ct: 2.00 to 2.39
2 1/2ct: 2.40 to 2.99
3 ct: 3.00 to 3.39
3 1/2ct: 3.40 to 3.99
4 ct: 4.00 to 4.39
4 1/2ct: 4.40 to 4.99
5 ct: 5.00 to 5.39
5 ct: 5.40 to 5.99
6 ct: 6.00 to 6.39
6 1/2ct: 6.40 to 6.99
Etc.