As expected, De Beers, in response to Rio Tinto's bid, said it was still committed to landing Ashton Mining and was considering its options,including whether or not to raise its bid.

It was on July 31, De Beers had made all cash hostile offer to acquire Ashton Mining's 40.1 per cetn in Argyle Diamonds valued at A$522.1 million (US$302 million).

Winning control over Ashton would secure marketing rights to Argyle Diamond's regular supply diamonds, in addition to its highly prized pink diamonds. Both Rio Tinto and De Beers secured 19.9 percent of Ashton through a deal with its 49.9 percent shareholder, Malaysia Mining Corporation Berhad (MMC).

Countering the De Beers' bid, Rio Tinto on Tuesday has offered A$1.85 per Ashton share against De Beers' A$1.62 per Ashton share valued at A$600 million (US$345 million).

In a statement released on Tuesday, Rio Tinto said the offer will include a scrip alternative, which the De Beers' offer did not contain.

Against De Beers' offer of 20 per cent premium over July 28 Ashton Mining share price on Australia Stock Exchange, Rio Tinto is offering the following-: A$1.85 in cash for each Ashton share held or one Rio Tinto share for every 15 Ashton Mining shares held , or; one Rio Tinto plc share for every 15 Ashton Mining shares.