Carat weight determines value in two ways:
First is the carat weight x price per carat = total of stone.
Second is the rarity factor in the relationship to the normal size range for a gemstone variety. Tsavorite is a good example: It becomes exceptionally rare and expensive is sizes larger than 5 cts. Another variety like Amethyst is readily available in sizes up to 50 cts., and therefore, has no rare size price increase.