The country's diamond-rich Marange field is one of the world's most productive diamond sources. Political and outside observers allege its output and profits are being under-reported by Mugabe, who has been in power for 31 years, The New York Times reported.
Mugabe's own Finance Ministry in Harare suggested some $60 million in diamond sales weren't reported last year, which rights group Global Witness alleged was thievery on Mugabe's part.
"The benefits of the diamond sales go primarily to allies of the president," said group spokesman Mike Davis.
Mugabe is running for re-election next year and opposition Member of Parliament Eddie Cross told the Times there was clear evidence profit-skimming was benefiting Mugabe and his party.
"It's quite clear that there's much more money floating around than is justified by the level of economic activity," Cross said.
He said government records showed $300 million worth of diamonds were reported to the government last year, while independent geological observers said one company alone had mined $1.4 billion of the gems.