A 46-carat stone it found in December recently sold for more than US$1mln.
The latest diamond, which was undamaged, will go on auction in March.
Chief executive Paul Bosma said: "The 70-carat stone was recovered in the northern, low-grade part of the pit where the bulk of our mining will take place in the coming months.
“Although the market for the smaller stones has been under pressure, we've seen continued demand and good prices realised for special stones.”
In production update last week the company said it saw a quarter-on-quarter drop-off in recoveries as it raked in 224,947 carats in the last full three month period versus 240,733 carats. However, it is sticking with its guidance for the full year of 820-870,000 carats.
The overall grade was 25.4 carats per hundred tonnes of ore.