London -The eight tons of gold, silver and copper unearthed from mines in Mongolia and Utah and now under guard at the Tower of London is the largest ever haul used to make Olympic medals."The medals arrived at the tower on 2 July, and we will keep them under tight security" Tracey Sands, spokesperson for Historic Royal Palaces, said.The 4 700 Olympic and Paralympic medals will be guarded alongside Britain's crown jewels until they are presented on the podium."For centuries the Tower of London has protected some of this country's greatest treasures so there can be no better sanctuary for the 2012 medals - the most precious possession any athlete could hope to possess," said London Mayor Boris Johnson.The winners' medals are certainly precious, even though gold only makes up a tiny portion of their alloy.A gold medal weighing about 410g contains only six grams of gold - 1.34% of its weight - the remainder being silver compound (92.5%) and copper.Heaviest, expensiveHowever, the recent gold and silver booms that have seen prices double since the 2008 Games in Beijing ensure that the medals are the most expensive in Olympic history.Added to this, the dimensions of the London medals (85mm in diameter and 7mm thick) make them the heaviest ever struck for the Summer Olympics.In Beijing, the medals were around half as heavy at 200g.But the London medals remain below the record set by the Winter Games in Vancouver in 2010, where the medals weighed up to 576g.British artist David Watkins designed the medals, which depict Nike, the Greek goddess of victory.The reverse bears the London Games logo in front of a radiating star motif, representing the spirit and tradition of the Olympics, and the River Thames, for the city of London."If there's the slightest blemish, we reject them," said Fergus Feeney, programme director at the 1 000-year-old Royal Mint, which produces Britain's currency and made the Olympic medals.Mongolia, UtahThe coveted discs were each processed 15 times in a huge hydraulic press known as the Colossus.The gold, silver and copper was extracted by Anglo-Australian global mining giant Rio Tinto from its Oyu Tolgoi plant in Mongolia and the Kennecott mine in Utah in the United States.The choice was controversial as its extraction techniques in Utah are considered by some to be polluting."Rio Tinto is not Olympic calibre in its behaviour toward its own workers and their families," said Ken Neumann, national director in Canada of the United Steelworkers trade union.At least the winning athletes can be assured the medals will be better than those issued the last time London hosted the Olympics in 1948.Then, post-war austerity meant that the medals were of poor quality and needed regular regilding.