Black diamonds are generally less expensive than colourless diamonds of the same size and quality for several reasons.
One is that while natural black diamonds are rare, they're not as rare as colourless
diamonds with exceptional clarity and colour. This difference in rarity drives the price down for black diamonds.
Another reason is that there’s less demand for black diamonds. Colourless diamonds have a higher demand due to traditional cultural associations and their sparkly appearance. Black diamonds, while gaining popularity, are not yet as widely desired.