The international diamond trade has been centred in the city since the 15th century. It is estimated that 85% of the global trade in rough diamonds passes through Antwerp, worth €43bn (£36bn) every year, equivalent to the GDP of Slovenia.

You would imagine that the crippling single currency crisis which continues to hold Europe in its grip would have had an icing effect on expensive ice, but business is booming, even as a quarter of Antwerp's young people struggle to find work.

We went to find out why, gaining access to one of the most secure buildings in the country guided by the entrepreneur Vashi Dominguez, who runs a successful UK-based diamond business from mine to retail.

It was fairly clear from the outset that news cameras aren't welcome in the diamond quarter. A police officer was dispatched to check our credentials after a CCTV camera filmed us on the pavement, while private security guards watched us warily from doorways.

After surrendering our passports, and with a prior appointment, we were allowed inside one 10-storey concrete building, in which trades valued at €1bn (£837,000) take place every month.

In a simple room with a series of substantial tables - and an even larger safe built into the wall - Vashi showed us three cut diamonds with a combined value of £2m as well as a scattering of smaller rough stones.

The four Cs still determine the price of a finished stone: cut, colour, clarity and carat (the weight, with a carat equivalent to one fifth of a gram), but the value of unusual, or "fancy" diamonds has been increasing dramatically at auction since the financial crisis began.

Vashi explains that as government bonds and currencies have become less attractive to investors since the start of the crisis in 2008, they have turned to valuable commodities like gold and gems.

"Prices are rising because demand is increasing. That's due to the slowdown and more interest from buyers in the east like China and India as well as other developing countries such as Brazil," he explains.

"There's another factor too: there has been a lack of major discoveries of new mines and some mines that have been discovered can't be built into viable businesses because the extraction process is so costly."

A massive new mine is being prepared in Canada, and De Beers continues to chip new diamonds out of Jwaneng mine in Botswana, but prospectors are working hard globally to establish new deposits.

The location of the current mines and trade patterns shifting eastwards could threaten Antwerp's pre-eminence as a diamond hub. More business could switch to Dubai, which is closer to southern Africa.

That's reflected in a change in the religion and ethnicity of the traders: the diamond quarter has been conspicuously Jewish, but more Indians are moving into the business, and into the area.

At the moment, Antwerp is still keeping its nose ahead of those rival cities looking to snatch its diamond tiara. It's an irony that the booming gem trade is based in a continent where economies have lost their lustre.