Space scientists revealed today that they have found the biggest known diamond in the universe - a 10-billion-trillion-trillion-carat gem.

US astrophysicists said they found the massive rock on Valentine's Day, positioned 50 light years away in the constellation Centaurus.

"You would need a jeweller's loupe the size of the Sun to grade this diamond," said Travis Metcalfe who led the researchers at the Harvard-Smithsonian Centre for Astrophysics.

The cosmic diamond is a chunk of crystallised carbon in the core of a white dwarf star.

It is 4,000 km across and weighs 2.27 million trillion trillion kg, which translates to about 10 billion trillion trillion carats, or a one followed by 34 zeros.