Perth-based Lonrho Mining says it has found a 131.5 carat diamond at its Lulo concession in Angola.
The rock was recovered from an alluvial bulk sample in which a 38.3 carat gem was also found.
The discoveries come ahead of a proposed drilling program by the company at the concession involving the testing of 61 targets.
Lonrho's managing director and mining veteran Miles Kennedy said the latest diamond recoveries underline the world-class exploration potential of the Lulo Concession.
"The recovery of such a spectacular 131.5 carat diamond in particular highlights both the prospectivity of the diamondiferous gravels in the Cacuilo River valley and, more significantly, the kimberlite pipe that has shed this stone," he said.
"Given the incredible size of this diamond, there is little doubt the kimberlite source is very close by."
The biggest rough gem-quality diamond ever found was the Cullinan Diamond at 3106.75 carats.
The Golden Jubilee, at 545.67 carats, is the largest faceted diamond in the world.
Both stones were found in South Africa and are housed with the Crown Jewels in London.Lonrho shares were off 0.1 cents, or 6.67 per cent, to 1.4 cents at 9.05am after emerging from a trading halt.