Mega-miner BHP has made a second unsolicited offer for rival Rio Tinto, but the offer again has been rejected.

BHP offered 3.4 shares of its stock for one share of Rio Tinto, up from its previous 3-for-1 proposal. Rio said the offer “undervalues” the company. BHP has not indicated whether it will sweeten its offer for a third try.

A merger would represent one of the biggest takeovers in corporate history, valued at nearly $150 billion, and create a mining powerhouse that dwarfs its rivals, including Anglo-American, part owner of De Beers.

BHP owns 80 percent of the Ekati Mine in Canada. Rio Tinto markets production from a variety of mines, including Diavik in Canada, Argyle in Australia, and Murowa in Zimbabwe. If the two merged, the resulting company would be the third-largest diamond producer and control 15–20 percent of the diamond market.

The bid raised speculation that Rio, or the combined company, might sell its diamond assets. Rio has denied prior reports that it planned to sell its Argyle Mine in Australia, where profits have fallen because of increased costs.

Bob Gannicott, CEO of Harry Winston (the former Aber), which owns 40 percent of the Diavik Mine in Canada, told the Rough Diamond Conference in Israel that his company had the right of first refusal to buy the remaining 60 percent of the mine and said his company would “like to” buy it.