Now the European Commission has proposed extending to diamonds a series of sanctions imposed on Russia since its full-scale invasion of Ukraine.
The EU has until now avoided a ban, largely because Belgium has been keen to protect the city of Antwerp: the diamond capital of the world.
In a corner of his office, Thierry opens a safe of colossal proportions and takes out a small white envelope, folded in half.
"There is a direct link between buying Russian diamonds and financing the war in Ukraine," says Filip Reyniers, director of IPIS - a research institute based in Antwerp.
He stopped importing Russian gemstones within a few weeks of the Ukraine invasion - which has resulted in higher costs for his business - and he believes the rest of the industry should have taken a firmer stance.
"Russian diamonds have come to symbolise war and human rights violations," said Belgian Prime Minister Alexander De Croo in September.
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