"Conservative estimates place the theft of Marange goods at almost $US2 billion since 2008," Partnership Africa Canada (PAC) said in a report that coincided with the start of an international diamond conference held to boost Zimbabwe's diamond sector.
PAC referred to diamond fields in eastern Zimbabwe.
The report singled out Mines Minister Obert Mpofu by saying that he had cleared mining concessions for security chiefs.
Advertisement"Far from defending the best interests of Zimbabwe, Minister Mpofu has presided over a ministry that has awarded concessions to dubious individuals with no prior mining experience, often under very questionable terms or circumstances," it said.
"He has solicited and approved applications from members of Zimbabwe's security forces, including those implicated in human rights abuses in Marange," the report added.
Zimbabwe's diamond industry has been tarnished by allegations of graft, and labour and human rights violations since Harare deployed security forces to drive away illegal miners from the Marange diamond fields.
Global watchdog Kimberley Process suspended exports from the area at the time, but lifted the ban after the government said it had pulled the security forces out of the area.
In an interview with the state-controlled newspaper The Herald ahead of the conference, Defence Minister Emmerson Mnangagwa denied the charges that diamond profits had been funnelled out of the country by people close to Mugabe.
But the PAC report 'Reap What You Sow: Greed and Corruption in Zimbabwe's Diamond Fields' said: "Hundreds of millions of dollars owed to Zimbabwe's treasury have been lost in both illegal and legal trades."
Finance Minister Tendai Biti, a member of the anti-Mugabe Movement for Democratic Change (MDC) party, complained in July that of the $US600 million in diamond revenues expected this year, only $US46 million had materialised.
"Many top securocrats loyal to" Mugabe and his ZANU-PF party were under scrutiny by PAC, one of the global natural resource extraction watchdogs.
But Mugabe blamed western sanctions on selected individuals and firms for the low revenue from the sale of diamonds.
"Due to the illegal sanctions imposed on the local diamond mining companies, the country has not been able to realize full benefits," he told the diamond conference, which took place in the resort town of Victoria Falls.
Mugabe said that a diamond law was in the offing "to further assure the global diamond industry that the government takes seriously good governance in the extraction and eventual trading of diamonds."
"Diamonds from Zimbabwe must... be allowed market space in order to trade competitively and fully benefit the nation," he added.
An industry expert told the same conference that Zimbabwe was selling its diamonds to few takers at rock-bottom prices as most foreign buyers shunned the country, which has been isolated by its former western allies.
Chaim Even-Zohar, president of the Tel Aviv-based diamond consulting service Tacy Limited, said that Zimbabwe has the potential to produce eight to 10 per cent of global gem production but was not benefiting fully as potential buyers were worried about trading with the country.
He said that while Zimbabwe complied with the Kimberley Process Certification Scheme, "major companies are scared, (and) insurance companies are afraid" of the US Office of Foreign Assets Control, forcing the country to dispose of its gems at 25 per cent less than their full value.