The Legacy, which was recovered from the Cullinan mine in South Africa in March, is a D-colour Type II gem quality diamond and is the second largest stone found at the mine to date.

CEO Richard Duffy said the sale was "significant" and an "endorsement of the quality not only of the Legacy diamond, but also the Cullinan orebody itself, which is known for its exceptional stones".

BMO's Edward Sterck said the sale price matched the bank's estimate for the sale value and was worth about $30 per carat for realised prices at Cullinan in the June quarter, boosting revenues and making the long-awaited transition to net cash flow positive more likely.

He noted realised prices at Cullinan had been pretty variable over time and "exceedingly weak" recently, with $94/ct reported in the December quarter, although this improved to $101/ct in the March quarter.

Sterck said the $30/ct value suggested "upside potential to our estimated realised price of $117/ct".

"Furthermore, with additional higher value stones likely to be sold in the quarter, such as the 210ct diamond recovered in April, pricing for Q4 looks likely to be stronger than forecast" he said.