It is alleged widespread human rights abuses took place in Marange
A human rights group says it is concerned about "continuing abuses" at diamond mines in Zimbabwe.
This follows a last-minute decision by Zimbabwean authorities to halt a three-day sale of about 300,000 carats of rough diamonds.
Global Witness says some mines remain in the hands of the military despite an agreement with international monitors.
Insiders have told the BBC that the sale was only halted after "blood diamond" trade monitors intervened.
"We're obviously pleased that this auction has been cancelled but overall we're still concerned about the situation in the diamond fields in Marange," Global Witness' Anne Dunnebacke told the BBC Network Africa programme.
Senior Zimbabwe's mines ministry official Thankful Musukutwa on Thursday told a news conference in Harare that the auction had been stopped because it had not been approved by the Kimberley Process (KP), set up to regulate the trade in "blood diamonds" - those mined in conflict zones.
"No export will take place prior to certification by the KP monitor," he told reporters.
Some 80% of sales from the planned three-day auction would have gone to the Zimbabwe government, according to reports.
Ms Dunnebacke said that while other diamond fields had been taken over by companies, she believed a "large proportion" of the Marange diamond fields remained under military control.
"The remainder of the diamond fields appear to still be under military control with the military still taking part in exploitation, the trading, the smuggling and of course the associated human rights abuses."
Zimbabwe went into partnership with two South African-owned firms to extract diamonds in October.
Last November Zimbabwe narrowly missed being suspended from the Kimberly Process and so banned from selling diamonds.
It promised to allow monitors to examine all shipments that come from the Marange mines and was given until June to clean up its diamond trade.
But Global Witness says those monitors are still not in place.
Investigators have found evidence of killings and forced evictions at the Marange field after soldiers moved in, saying they were acting against illegal miners.
Activists have accused the military of carrying out widespread atrocities in the mines and say the profits from the stones go to President Robert Mugabe and his allies.
The government has always denied these allegations.