Firestone said it recovered the stone undamaged and it would go on sale at the next tender scheduled to take place at the end of January 2019.
CEO Paul Bosma said the recovery was "good news" as the company started mining across the pit towards the northern side of the orebody.
The market agreed, with shares up 14.07% to 3.85p (US4.85c).
Firestone said it had also noticed more consistent pricing for smaller stones at the December 7 sale, which confirmed the trend reported by other producers.
"We are also pleased by the fact that the prices of our smaller, lower value goods have stabilised and we look forward to further improvement as we head into the new year on the back of an expected positive diamond jewellery retail season," Bosma said.