Black diamonds are real (It's still a pure carbon crystal, so called "Carbonado", but it is technically a diamond).They don't sparkle too much unless they are of AAA nature, because due to the fact they are coated black, they absorb light rather than refract it.
They are real diamonds, but they have a different crystalline structure. The regular crystal structure of the diamond is what causes the sparkle: light comes in, bounces around in a predictable way, and is bounced out.
Black diamonds have a "polycrystalline" structure, which means it's kind of like many diamonds smashed together. Since they're all at different angles, and so you can't carve it to make sparkles. Instead, the light just gets absorbed, making it black.
An Interesting take on the origin of Black Diamonds
A team of U.S. geologists have published evidence relating to a different origin of these black diamonds: interstellar-outer space. They have found that black diamonds contain fine elements of nitrogen and hydrogen which they claim are sure indicators of an extraterrestrial origin.
The study by renowned researchers found that the chemical properties of carbonado indicated that the mineral formed in a supernova explosion that took place prior to the formation of our Solar System.
In this sense, Carbonado is found similar to carbon-rich cosmic dust, likely formed in an environment near carbon stars. The diamonds were eventually incorporated into solid bodies that subsequently fell to Earth as meteorites.
