Actually the post is in the right section, The white dwarf star
BPM 37093 is believed to have a core of diamond, 4000km in diameter.
It weighs 10,000,000,000,000,000,000,000,000,000,000,000 carats, your 530 carat diamond pales in comparison, eh shygrneyzs?
Beat that!
(It's not a planet though... There's no reason a planet can't be made of diamond though.. but it's very hard to work out what planets outside our own solar system are made of, as the spectrum of the star floods the signal from the spectrum of the planet, I doubt that a planet made of diamond has been discovered).
I guess i'm trying to say "LowTax4Eva, WHY should a planet be different from a diamond?"