If people here on earth already fight over diamonds, imagine what they would do over one the size of the planet itself. That might actually be what David Kaplan and astronomers from University of Wisconsin-Milwaukee have discovered, a dead star so cold that it might have crystalized over to become a literal gem in the sky.
Instead of going supernova, a dying star cold also collapse on itself to form extremely dense celestial bodies called white dwarfs. These are mostly composed of oxygen and carbon and cool over billions of years, making them hard to detect even by the strongest astronomical equipment. In fact, the discovery of this giant diamond was only a by-product of the discovery of PSR J2222-0137, a pulsar, that is, a rapidly spinning neutron star made up of the remains of a supernova, that is the gravitational companion of this white dwarf.
When PSR J2222-0137 was discovered, it was observed to be bound to a companion star. At first, the scientists theorized that it is also another pulsar. However, after observing the behavior of the two bodies over a period of years, they concluded that the second object could not have been a neutron star because their orbits were too constant, orbiting each other once every 3 days. The only other possibility is that this was a white dwarf, which turned out to be a very special one at that.
A white dwarf isn't that uncommon. A white dwarf with a neutron star companion isn't that rare. Theoretically, the scientist should have been able to see a white dwarf that was 100 fainter than any white dwarf-neutron star combo or one that was 10 times fainter than any known white dwarf. The thing is, they didn't see anything at all. The only conclusion they could reach is that the white dwarf is extremely cold, 4900 °F or 2000 °C according to their calculations. That might not be "cold" by human standards, but for a star, even a dead one, that is way below freezing.
Due to this, the astronomers believe that the white dwarf is practically made up now of crystallized carbon. In short, a giant diamond. Theoretically, such things are not rare in space. But, because they are really faint, they're almost impossible to find and it is only because of the discovery of PSR J2222-0137 that we were able to discover one. But before you go dreaming up of space jewelry, PSR J2222-0137 and this white dwarf is 900 light-years away, so unless humans are able to develop Faster Than Light space travel, don't even dream of getting a chunk of this thing.