With a beaming smile, the jewellery shop owner says they are uncut Burmese rubies. “I can get some others if you want,” she says.

Verifying her claims is difficult, but that is the task of US customs officials following sanctions imposed by Washington on Burma and its ruling military junta.

In theory, the ban, which US officials began enforcing in October, should have a significant impact on the trade in gemstones, particularly rubies, since Burma is the world’s largest producer. In practice, however, some jewellers and traders see Washington’s clampdown as futile.

The scepticism is prevalent in Thailand, the world’s largest trading centre for coloured gems. From his office on Silom, the Bangkok road that is a centre for gem traders and jewellers, Somchai Phorchindarak, chief executive of the Bangkok Gems and Jewellery Fair, says the US is missing its intended target, punishing instead small Burmese miners and farmers who search for gems, and the cutters and manufacturers who turn them into jewellery abroad.

Still, Mr Somchai says the 1,500 companies in his association, of which 90 per cent are exporters, have to follow Washington’s demands and stop dealing with Burma. “We have a good reputation and don’t want to take any risks,” he says. Such pre- occupations have prompted some luxury brands, including Cartier and Bulgari, to shun Burmese stones ahead of the US sanctions.

Burmese gems are still sold by leading auction houses and luxury retailers. In private, many argue that Burma’s importance to the gem industry is too long-standing to be halted by the US. “There are simply too many stones from Burma already on the market, too many trading channels, both legal and illegal, for this to stop,” says one trader.

Burma is a world leader in high-quality jade and rubies and developing alternative markets could raise new ethical problems, some experts warn. Emmanuel Ouachée-Feldmann, an independent expert, voices concerns about Tajikistan, where rubies are mined near the Afghan border. “To avoid Burmese stones, we are going to help boost the value of gems that might well already be controlled by Islamic extremists,” he says.

Ronald Abram, whose eponymous business is one of Asia’s leading jewellers, also draws a distinction between African blood diamonds and Burmese rubies. “All the very important rubies come from [family] vaults,” he says. “When it comes to blood diamonds, it’s our responsibility and we know what’s happened. Those [Burmese] generals are dictators but nobody has proven to me that they butchered people for the rubies.”

The biggest challenge for the US ban is likely to be its enforcement. While the diamond industry has put in place the so-called Kimberley Process to track where diamonds are mined, following a public outcry over diamond-funded wars in Africa, no such system exists for coloured stones.

“There has to be a proper and new process of certification,” says an executive of Thien Po, a Thai exporter of jewellery, 60 per cent of which goes to the US. “This ban will just make the trading of the gemstones more difficult to control.”

In the meantime, there are only a handful of laboratories worldwide with the testing facilities to determine the origin of rubies.

But the sanctions have won over some in the industry. Jewelers of America, a trade group for retail jewellers, says customer needs can be met without Burma. “While Burma is traditionally the home of the finest rubies, there are several other ruby sources,” says the group.