The 0.5-2m thick layer was discovered by excavating below a “false bottom” of basement-like clays and so far has yielded 798 gem quality diamonds weighing a total of 98.3 carats from 380 tonnes of screened gravels.

“It’s most unusual, but it’s there and it’s producing gem quality diamonds” - Kennewell

In the 18 months Cluff has been working the deposit it has increased the average grade from 3 carats per 100t to 25 carats/100t, said managing director Peter Kennewell. The largest stone recovered so far was a 1.1 carat white, together with another two stones of about 1 carat in total weight.

“The company’s main aim at the moment is to determine what sort of geological structure it is dealing with,” said Kennewell.

“The indications are that it is a volcanic lake deposit formed down the throat of a vent. It seems diamond-bearing gravels were blown up as some type of geyser into the lake deposits.

“It’s most unusual, but it’s there and it’s producing gem quality diamonds.”

The next step for Cluff is to bring in a drilling rig to try and quantify the resource, which it hopes to do within the next few months.