It is the largest stone recovered so far this year and follows the recovery of a 46ct gem diamond in December, which sold for over US$1 million at a recent sale completed February 1.
CEO Paul Bosma said the diamond had been recovered in the northern, low-grade part of the pit where Firestone would be carrying out the bulk of its mining over the next few months.
"Although the market for the smaller stones has been under pressure, we've seen continued demand and good prices realised for special stones," he said.
The 70ct diamond will be sold in the next tender in March.
Firestone previously said an equipment failure in the mine's plant in January was expected to impact tonnes treated and carats recovered in the March quarter.