A diamond of 18.39 carat was discovered at the Schmidtsdrift mine of Lonrho Mining, the company said on Tuesday.
It said the "exceptional" stone was a fancy yellow perfect octahedron, similar to a 61.30 carat diamond found last month, and was expected to yield a high value.
"We are pleased to see that Schmidtsdrift is once again producing large stones," Lonrho chief executive Charles Mostert said in a statement.
"These discoveries are due to a recently completed organisational restructure at the alluvial mine."
The stone of 61.30 carat was recently sold for $754 321 (about R1,5-million), amounting to $12 305 per carat .