Petra said the stone is one of the 20 biggest, high quality diamonds to have been found

The diamond weighs just over 100 grams and could be worth more than $20m

One of the largest, high quality diamonds discovered to date has been found at a mine in South Africa, mining group Petra Diamonds has revealed.

The 507 carat stone, which could be worth in excess of $20m (£12.5m), was found with three other large diamonds.

They were discovered at the famous Cullinan mine, where the largest diamond in history was found more than 100 years ago.

The new find is being analysed by experts to determine its true value.

A 480 carat diamond found at the end of last year fetched $18m.

"The Cullinan mine has again given the world a spectacularly beautiful and important diamond," said Petra's chief executive Johan Dippenaar.

"Initial indications are that it is of exceptional colour and clarity, which suggest extraordinary potential for its polished yield."

Petra said the stone is one of the 20 biggest, high quality diamonds to have been found.

A 168 carat stone was also discovered, alongside one of 58 carats and another of 53 carats.

The largest diamond to be discovered, named the Cullinan, was 3,106 carats. It was cut into nine separate stones, many of which are in the British Crown Jewels.

Paris was so keen to impress ‘The Hills’ star

The blonde socialite threw the surprise party for Doug Reinhardt at her Beverly Hills mansion on Saturday (17.10.09) and wore a sexy hot pink outfit and a black diamond mask as she burst out of the giant-sized sweet treat.

A source told the New York Post newspaper: “Paris went all out for her man’s birthday. No expense was spared. He had no idea about the party and was so happy.”

Paris was so keen to impress ‘The Hills’ star – who she reunited with in the summer when he whisked her away on a romantic break to Fiji - she organised for rap group Three 6 Mafia and hip-hop star Ya Boy to perform for partygoers.

Guests including Hollywood beauties Hayden Panettiere and Kristanna Loken, partied until 3am at the nightclub Paris has built in her home, while home videos of the couple were screened throughout the mansion.

Doug is not the only star to be presented with a giant cake on his birthday.

When Hollywood actress Katie Holmes turned 30 last December, her husband Tom Cruise surprised her with an enormous five-tiered sponge.