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 .
The Schmidtsdrift mine, of which 80 percent is owned by Lonrho Mining and 20 percent by the Schmidtsdrift Communal Property Association, comprises six contiguous farms along the western bank of the Vaal River. - Sapa