Where do black diamonds come from? We don’t actually know.
Diamonds, as I’m sure you know, come in many different colours. All of the colours except one are found in igneous rocks like kimberlite, lamproite, or lamprophyre, and we have a good idea how they formed. Roughly, they were formed by incredible pressures deep down – in or near the mantle, hundreds of kilometres underground – and are then carried upwards by igneous rocks that are bursting towards the surface. As they form, diamonds can pick up other elements that change their colour: nitrogen leads to yellow or brown diamonds; boron gives us blue diamonds. If they get a good dose of radiation they can turn green.
Black diamonds, also known as carbonados because they’re coloured by incursions of graphite and other types of carbon, are different. They’re never found inside igneous rocks, never found in the same places as all the other diamonds. They’re found in sedimentary rocks, and only in two places: Brazil and the Central African Republic. And we are not actually entirely sure why.
There are theories, of course. Maybe they were formed the same as other diamonds (boooring). Maybe they were formed by a particularly strong meteorite – meteorites tend to produce microdiamonds as a result of the intense pressures and temperatures of the impact. Maybe spontaneous terrestrial radiation provided the necessary temperature. Maybe, and this one is a long shot, black diamonds were actually formed by a supernova and crashed to Earth as part of a meteorite 2.3 billion years ago. That would explain why they’re only found in Brazil and the Central African Republic: 2.3 billion years ago those two places were next to each other, both part of the supercontinent Columbia. This last idea has some good evidence attached: trace elements inside the black diamonds of Africa and South America match up, suggesting a common origin.
[Thanks to Gareth E. for suggesting this topic.]