The good news, black diamonds are real! The bad news, they don’t sparkle, because they’re black. They absorb light rather than refract it.
They are real diamonds, but they have a different structure. A white diamond sparkles because light comes in, bounces around in a predictable way, and is reflected out. The cut of the diamond is designed allow light to escape in the most ideal way.
Black diamonds have different structure what equates to having many diamonds compressed together. Since they’re all at different angles, and so you can’t cut it to make sparkles. Instead, the light just gets absorbed, making it black.
Jewelry stores mostly sell them at about the same price as white diamonds, but they don’t sell nearly as well because they’re not attractive. They used to be cheaper than white diamonds, but aggressive marketing has driven up the price to unrealistic levels.
It is recommended that you don’t pay white diamond prices for a black diamond, they DO NOT hold their value.