This one is easy: the bigger the diamond, the more it’s worth and the more it costs (all other factors being equal). Diamonds are often listed with a per carat price, especially when buying on the internet or wholesale. It makes sense, therefore that a larger diamond would cost more. However, an additional factor is the rarity of large diamonds. This makes the increase in price exponential. For example: a VS2, G color diamond, good cut 1 carat diamond might cost $5000. You might then expect that a 2 ct. diamond of the exact same grade and cut would cost $10000. But it doesn’t. It costs more like $20,000. This is because 2 ct. diamonds are much rarer than 1 ct. diamonds, and so forth. Another element of carat weight that can affect its value is the convenience of its size. Most jewelry settings are designed to fit stones of standard sizes (e.g. .5 ct, 1 ct., 1.5 ct., etc.) Therefore, you might be able to get a deal on a .87 carat diamond, for example.
Learn More About Diamonds
How COLOR affects the value of a diamond
How CUT affects the value of a diamond
How CLARITY affects the value of a diamond
Diamond Imitations and Controversies