The company said it had found a 55-carat diamond at its 75% owned, five-hectare Baoulé kimberlite pipe in the west African country.
Stellar said the find confirmed the presence of large stones in the pipe and it was continuing to recover high-quality gems of up to 12 carats.
The gem - the largest discovered to date at Baoulé - was an unusual stone as it had a low-quality coating but a seemingly better quality diamond inside, Stellar said.
It added that the quality and potential value of the stone was highly uncertain and it would carefully analyse it to find out how much it was worth before deciding on the sales process.
Chief executive Karl Smithson said: "We are extremely encouraged to have recovered this 55-carat stone as it confirms that the Baoulé pipe is a possible source of the large diamonds mined in alluvial deposits downstream of the Baoulé pipe for many years, several of which have been +100 carats in size.
"With approximately 63% of our trial mining and processing at Baoulé now complete, subject to the company having sufficient working capital, we're on track to finalise the trial mining and processing of 100,000 tonnes in the third quarter and look forward to providing further updates in due course."
However, the group said the average grade of the 8,043 carats recovered to date from Baoulé was slightly lower than previously reported. The stock fell 0.7p, or about 7.4%, to 8.8p after the update.