A commodities trader has launched legal action after a $36 million consignment of copper was allegedly swapped for spray-painted stones before arriving in China.

Last summer, Switzerland-based company Mercuria Energy Group agreed to buy around 10,000 tons of copper blister, an impure form of the metal, from a Turkish company.

But before it was shipped to China from a port near Istanbul, the company says the copper was replaced with paving stones that had been spray-painted black to look like the metal.

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About 6,700 tons of the material was loaded for shipment in containers, but it was only when the cargo was opened in the port of Lianyungang that Chinese inspectors discovered the switch, according to Mercuria's lawyer Sinan Borovali from KYB Law.

He added that the company had already paid for 90 percent of the material loaded for shipment.

Mercuria, one of the five-biggest oil traders in the world, has opened up legal cases in Turkey and the UK against the Turkish copper supplier Bietsan Bakir.

The company said that so far 13 people had been taken into custody as part of the police investigation.

"We are aware and closely following up with the latest news and developments in Turkey regarding the ongoing legal proceedings and investigations," Mercuria said in a statement. 

"Unfortunately the news somewhat reflects the truth and we have incurred a loss of more than 6,000 metric tons of copper blisters that we purchased due to substitution of cargo."

The company thanked the Istanbul Financial Crimes Department for its investigation and reiterated its "utmost trust in the Turkish justice system."   

In general, fraud of this kind is stopped by official seals being fixed to containers so they can't be opened. However, the KYB lawyer said the fraudsters had replaced them with fake ones to hide the crime. 

"There has been a criminal investigation petition by the buyer against the seller and two intermediaries," Turkish police said in a statement to Bloomberg. "It's been determined that the incident is the outcome of fraud perpetrated in an organized manner."

Traders can usually make a claim against a cargo's insurance policy if there is a non-delivery, but Mercuria discovered only one of seven contracts used by Bietsan Bakir to insure the cargo was legitimate. The rest were forgeries.

Bietsan Bakir has not responded to requests for a comment.