(IDEX Online) - Surat's long-awaited bourse - a vast $355m building with 4,400 offices - is finally due to open in September. The Surat Diamond Bourse (SDB) says it will become the world's biggest diamond trading hub, with an estimated annual turnover in excess of $25bn. It was supposed to open in January 2020, according to its own website.Surat has long been the capital of diamond manufacturing, handling over 90 per cent of all the world's gems, but the trade has been centered in Mumbai, a five-hour drive away.SDB will challenge the dominance of Mumbai's Bharat Diamond Bourse currently the world's largest diamond bourse and home to around 2,500 traders. The allocation of office space is to begin in February."It is going to be a win-win situation for the diamantaires in Surat," SDB spokesman Mathur Savani told the Times of India newspaper. "From the manufacturing hub, Surat will become the diamond trading hub of the world." The new bourse in Surat will be the world's largest office building, with nine 11-story towers and covering a site larger than The Pentagon in the USA.The Surat Diamond Bourse pictured under construction