ANTWERP, Belgium — The bond between banker and diamond dealer has long been a cozy one in an industry shaped by an insular culture of carats and cash, secrets and intrigues, weddings and bar mitzvahs. But lately those old bonds are dissolving in the packed square mile of gem traders here that forms the hub of the global diamond industry.

A flurry of legal cases, investigations and leaked bank documents have drawn attention to the opaque movement of diamond-backed money. The dealings between gem traders and bankers are coming under new scrutiny as European governments extend crackdowns on tax evasion and money laundering at a time of persistent economic trouble.

The complex ties between Antwerp’s bankers and its diamond merchants have investigators especially worried. Government authorities, they warn, are easily outpaced by an industry skilled at moving vast amounts of money through international transfers, backed by diamonds or offshore cash as collateral.

“Our investigative resources are like a Citroën economy car while our targets are flying and using jets,” said Jean-Claude Delepière, director for the past 20 years of the Belgium Financial Intelligence unit, an agency that sorts through reports of suspicious transactions for prosecutors. “We can’t be naïve, because the interests are enormous.”