TONY EASTLEY: Elizabeth Taylor eat your heart out. Scientists believe they've discovered the largest diamond known to mankind, estimated at around ten billion trillion trillion carats.

Problem is for all those diamond-fanciers out there, the gem in question is around 50 light years from Earth.

Astronomers believe it's the super-compressed heart of an old star, which has burnt out and become a 1500-kilometre-wide lump of crystallised carbon. One big rock!

Nick Grimm reports.

NICK GRIMM: Its official name is "bpm 37093", but in light of status as the largest diamond ever discovered, the astronomers who spotted the cosmic gem decided something a tad more evocative was required. So the scientists at the Harvard-Smithsonian Centre for Astrophysics settled on "Lucy", as in the Beatles song Lucy in the sky of diamonds.

VINCE FORD: Once again nature turns them out bigger than anything we can do.

NICK GRIMM: And let's face it, big diamonds just set imaginations alight as Vince Ford from Canberra's Mount Stromlo Observatory points out.

VINCE FORD: This could be quite an interesting one just from the point of view of a life story of stars as well as the potential diamond mine.

Actually this would probably be the oldest stage of a star's life that we have ever looked at because of course once they go past that and are totally cool, not emitting any radiation at all, you can't see them.

NICK GRIMM: The statistics on Lucy are impressive, it's estimated to be a cool ten billion trillion trillion carats. By comparison, those here on Earth are mere baubles. The biggest ever found here was a very modest 3,100 carats.

Lucy meanwhile is 1,500 kilometres from end to end and that's an awful lot of engagement rings, necklaces and earrings.

VINCE FORD: You can see every female listener's eyes going into orbit as you give that carat figure.

NICK GRIMM: Lucy's sheer size might be dazzling, but it's what's known in astronomer's parlance as a white dwarf, which is the left-over of a burnt-out star once much like our own sun.

Vince Ford again.

VINCE FORD: So what's happened is that it's hit the stage where it's run out of heat, it's left there as a big pile of cinders and metal and because of that high mass, the pressure in the core of it, just gravity pulling the star tighter and tighter, has compressed the core down to the stage where the carbon in it has actually crystallised and of course what you get when you crystallise carbon under high pressure is diamond.

So here you have it, the world's biggest artificial diamond, or the universe's I suspect.

NICK GRIMM: Of course Lucy is probably well out of reach of most people, after all it's 50 light years from Earth for a start, and while that might still be relatively close in astronomical terms, it's unlikely to spark a space race to claim possession just yet.

But as Vince Ford points out, Australia is already well-placed to claim it as our own.

VINCE FORD: This huge DeBeers thing is sitting right down in the Southern sky in the constellation of Centaurus, just near the Southern Cross. All the good things seem to be in the south.

TONY EASTLEY: Picture yourself on that one. Astronomer Vince Ford from the Mount Stromlo Observatory in Canberra, with Nick Grimm with that report.