A diamond's price increases exponentially with its carat weight. A 1.00 carat diamond is much rarer and therefore significantly more expensive than two diamonds weighing 0.50 carats each. The reason is simple: the larger a rough diamond is, the rarer it is in nature. Small diamonds, often called 'melee', are generally valued per carat but at a lower price per unit than large stones.

Carat measures weight – not diameter. How large a diamond appears also depends on its cut shape and proportions. A well-cut round brilliant of 1.00 ct typically has a diameter of about 6.5 mm, while one with deeper proportions of the same weight can look noticeably smaller. In the trade, 'magic sizes' are discussed – exactly 0.50, 1.00, 1.50, and 2.00 carats – because these weight thresholds are psychologically significant in the market and often produce a disproportionate price jump. A stone at 0.98 ct can cost noticeably less than an identically graded stone at 1.00 ct.

Diamond transactions divide stones into weight categories with very different pricing:

• Melee (under 0.20 ct): Side stones in rings and earrings. Rarely individually certified – valued in bulk on total carat weight.

• Small solitaire (0.20–0.49 ct): Popular in simpler jewelry. Sometimes certified by GIA depending on quality.

• Half carat (0.50–0.99 ct): Common size for engagement rings. Good balance between visible size and price.

• One carat and above (1.00 ct+): Price per carat rises dramatically. A certified 1.00 ct G VS1 can cost three times more per carat than a 0.50 ct of identical quality.