Old river sediments in Brazil and the Central African Republic contain minerals found nowhere else on Earth; a form of diamond, dark black or green-gray, mixed with a lot of graphite (the stuff inside pencils) and often with small bubbles inside.
These diamonds are unusual for a variety of reasons, they make up some of the largest diamonds ever found and yet the origin of them is unknown. Most diamonds come from kimberlites; volcanic tubes which shot diamonds up from the Earth’s mantle up to near the surface (in some areas diamonds are mined from kimberlites, in others, they are found in rivers downstream of them). Carbonado diamonds don’t have any signs of kimberlites associated with them they also don’t include any material from the Earth’s mantle, which can often be found trapped inside diamonds.
Diamond is formed at high pressure and temperatures and so doesn’t often have any pores or bubbles within them, but these types of diamonds are often very porous (full of holes). The type of carbon which makes them up is also unusual. Whilst all diamonds are made of carbon, carbonados are made of a lighter form (isotope) of carbon than other diamonds found on Earth. Dating has shown them to be at least 3.8 billion years old, half a billion years older than the oldest known kimberlites adding to the mystery of their source.
Their two geographical locations can be easily explained by plate tectonics, whilst currently, separate the two were at one stage part of the same continent. (Later geological processes affecting more the countries to the west of CAR in Africa). This point source location, unusual isotopic composition, and the high-pressure high-temperature but also high gas pressure (allowing all the bubbles to form). Narrow down the possibilities of how these diamonds formed and suggest an extraterrestrial origin.
The most exciting hypothesis is that these diamonds formed in a supernova; the exploding death of a star, this would explain both the carbon isotopes and also the bubbles. So whilst Thor’s hammer might have been forged from a neutron star, these diamonds found on Earth may come from the remains of an exploding star. Other theories include being knocked off a white dwarf star or coming from one of the giant planets in the solar system or even from an exoplanet. This is an area of active research and further understanding of the processes going on in these high-pressure temperature environments will help narrow down the candidates but it’s amazing to think that these diamonds may have come from outside our solar system.