If you're getting married, and your partner tells you that they want the biggest diamond you can find, they might live to regret it. As of 2011, we can confidently say that the biggest diamond we're aware of is five times the size of the entire Earth.
Situated around 4,000 light years from Earth, one of the stars that makes up the constellation of Serpens - a star named PSR J1719-1438 - has a curious companion; a comparatively tiny planet that circles around it in an intensely close orbit. Scientists believe that this is all that remains of what was once a companion star to PSR J1719-1438, which siphoned off all of its outer layers over millions of years leaving it as a tiny white dwarf. Now all that remains is an outer layer of oxygen, and its core of pure carbon, which we can tell from its mass is crystalline and almost certainly diamond. Getting close enough to mine it might be an issue - it's so close to the parent star that its year is only two hours long.