(IDEX Online) - The vast new Surat Diamond Bourse (SDB) - stretching across nine 15-story towers - has become the biggest office building in the world.It has over 7.1m square feet of floor space, compared with just over 6.6m at the Pentagon, HQ of the US Defense Department.The bourse, built at a cost of $388m, is due to be officially opened by India's Prime Minister, Narendra Modi, in December or January and the first businesses are due to move into the bourse, in south Surat, in November.SDB has shared new interior pictures of the completed bourse on its website.The project's CEO, Mahesh Gadhavi, told CNN that the size of the building was dictated by demand from cutters, polishers, and traders, and that the 4,700 offices and workshops had all been bought before construction.He said that despite covering 35 acres, no office was more than seven minutes away from the main entrance.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.Surat has long been the capital of diamond manufacturing, handling over 90 percent of all the world's gems, but the trade has been centered in Mumbai, a five-hour drive away.Pics courtesy SDB