Global diamond giant De Beers yesterday challenged Ashton Mining to accept its $522 million takeover, describing its bid as "a closing offer" which fully valued the company.

De Beers' managing director, Mr Gary Ralfe, described the rise in Ashton's share price to 10 cents above his company's $1.62 bid price as a "bit of froth" which he expected to evaporate in the absence of a higher offer.