Diamond Pricing: Found more often, yet still rare. Weighing 400 milligrams, a Round Brilliant Cut has a diameter of 8.2mm. The prices for 2 carat diamonds range between $5,060 and $48,400 per carat. Compared to other diamonds, 2 carat stones are found more often, yet they are still rare: 1 in 1 million diamonds are quality 1 carat stones, and only 1 in 15 million stones are 2 carats.