DIAMONDS, more than being a girl's best friend, are Belgium's most important export to Hong Kong. The territory spent $6 billion last year, or 60 per cent of the volume of trade between Hong Kong and Belgium, on the twinkling, sophisticated gems. Only the United States bought more diamonds, said Belgium Trade Commissioner Andre Mathieu. Last year, when the world market suffered a slump, only Hong Kong increased the volume of diamonds it bought, and, in so doing, pushed Japan back to third place as an importer of the precious stone, he said. Belgium exported 20 per cent of its polished diamonds to the territory last year. More buyers use the Belgian market because the stones sold were ''larger and of a higher value''. Antwerp, which is the world diamond centre, handles more than 50 per per cent of all polished diamonds produced and is the international pivot for trade in rough and industrial diamonds. The city sells more than 500 billion Belgian francs (about HK$110 billion) worth of diamonds each year, or seven per cent of Belgian exports. This year, the city has been named both Cultural Capital of Europe and Diamond Capital to highlight its importance to the industry. More than 20 special diamond shows, exhibitions, art and jewellery events and trade fairs have been held this year. The programme will continue until the last day of the year. The Diamond High Council, Belgium's official representative and professional co-ordinating body of the Belgium diamond trade and industry, has its headquarters in Antwerp. Antwerp's Diamond Centre is concentrated on one square mile near the city's Central Station, in which there are 1,500 diamond firms and four bourses - keeping competition keen and prices low. Mr Mathieu said most of the diamonds Hong Kong dealers bought wholesale at the Antwerp bourse were raw stones that were destined to become fine jewellery. The territory's dealers bought for different markets, in particular South Korea, Thailand and Taiwan, Mr Mathieu said.