text: They are supposed to be a girl's best friend - or at least her most valuable. But the diamond ring appears to have lost its sparkle according to experts who claim that women are abandoning the traditional jewel in favour of more exotic gems. The huge increase in demand for coloured stones, such as sapphires and rubies, has seen their prices soar. Scroll down for video . Kate Middleton proudly wore a Ceylon sapphire once owned by Princess Diana as her engagement ring . And experts say the trend might prove that diamonds are not forever after all, as the value of the jewels has been vastly overtaken by more fashionable stones. Demand has seen the value of some coloured stones increase by more than 2,000 per cent over the last ten years, with leading auctioneers now branding them a better investment than diamonds. Celebrities such as Kate Middleton, who proudly wore a Ceylon sapphire once owned by Princess Diana as her engagement ring, have driven the craze. A-list stars Beyoncé, Naomi Watts, Rosamund Pike and Mila Kunis have also played their part by flashing their beloved coloured gems on the red carpet. The number of women picking their own engagement rings is also thought to have had an effect, with men always likely to choose a traditional diamond. While diamonds are still broadly worth more money than softer stones such as emeralds, the increase in price generated by coloured stones in recent years has far overtaken diamonds. A-list stars Rosamund Pike by flashing beloved coloured gems on the red carpet . As the gems are much rarer than diamonds the surge in demand - especially from Asia - has played a significant part in driving up their cost. The surging popularity of coloured stones is one of the reasons leading auctioneers Bonhams include them in most sales around the world, alongside diamonds. Its auctioneers were delighted when a pair of 1930 Cartier art deco ruby and diamond brooches recently sold for £482,500, far outstripping the presale estimate of £80,000 - £120,000. A single-stone ring by Van Cleef & Arpels featuring a Burmese ruby weighing 13.34 carats, sold for £350,500 to a Hong Kong buyer, breaking price-per-carat records in the UK last year. Jean Ghika, head of jewellery in the UK and Europe at Bonhams, said that as well as being fashionable colured stones are also becoming seen as a sound investment. She said: 'There is a new move towards colour. Women are bolder in their choices and looking at alternatives to diamonds when it comes to engagement rings and everyday jewellery. 'Women are also in constant pursuit of something truly original, and a coloured stone, where every example is different they are able to have something unique. 'As ever, fashion is partly celebrity-led and we're seeing some of the world's biggest stars looking stunning in coloured gemstones on the red carpet.' Actress Mila Kunis also played her part, seen here wearing ruby jewellery to the Jupiter Ascending premiere . A ring featuring a Sri Lankan sapphire weighing 30.08 carats was recently sold by the auctioneers for £326,500. Bonhams said that a near identical ring sold a decade earlier for only £11,950, equating to a 2,200 per cent increase in price-per-carat. Miss Ghika added: 'Diamonds are typically seen as the more 'robust' area in the gemstone market in terms of seeing a return on investment. 'But figures over the last decade demonstrate that coloured stones have actually overtaken diamonds in terms of growth.' A new world record for the most expensive ruby, regarded as the most valuable coloured gem, was set last year when a 8.62 carat Burmese stone sold for £5.4million.
target: Increased demand for coloured stones has seen their prices soar .
  Value of diamonds has been vastly overtaken by sapphires and rubies .
  Auctioneers are now branding them a better investment than diamonds .
  Celebrities such as Kate Middleton have driven the craze .