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 .
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 .