An extra license has been added to the recently-signed option deal with Mozambique firm, Morminas, a unit of the EIP Group of Portugal, Botswana Diamonds said.
As a result, three license blocks are being appraised on the Save River in Mozambique near the Zimbabwean border.
The Save River runs south-eastwards and drains an area of Zimbabwe including the Marange diamond region.
"Botswana Diamonds' objective is to explore the potential for alluvial and eluvial deposits which may have washed down from Marange," the company said in a statement.
Botswana Diamonds has a half-year exclusivity period in which it will review the data on the licenses and carry out preliminary exploration. Work has already started and a site visit slated.
Botswana Diamonds will negotiate a long-term agreement with Morminas if the analysis is positive.
Chairman John Teeling said: "Acquiring this additional licence increases the scope of our prospective play close to the Marange diamond fields in Zimbabwe. The experience and knowledge gained already in Zimbabwe will be invaluable in identifying diamond prospects in the Save region. Our geologists are now reviewing all available data, and given the right results, Botswana Diamonds will negotiate a long term agreement. "