A gem of a discovery

A tourist wandering in an ancient volcanic crater in Arkansas discovered a 4.38-carat diamond that lay on top of the soil catching the light of the morning sun.

Noreen Wredberg said she spotted the gem just off one of the paths in what is billed as the only diamond mine that is open to the public.

Visitors to Crater of Diamonds State Park pay $US10 ($14) to search a field strewn with rocks and gems that were vomited up during an eruption about 100 million years ago.

It has not yet been valued but uncut stones of more than three carats could be worth several thousand dollars.