(IDEX Online) - Caravanner Carmel Ash found a 424-carat yellow sapphire, valued at up to $200,000 (AUS$300,000), while "specking" - looking for gems, especially after a downpour - in Australia.She said she was "absolutely astounded" when what initially appeared to be a black rock turned out to be a huge and valuable gemstone.Carmel, 71, who describes herself as "half-blind", was walking with husband Mervyn at public "fossicking" - or rummaging - area near Willows, in central Queensland. "It's a bonus we weren't expecting and as retirees it's just going to be a big top-up for us," she told ABC, Australia's national broadcaster.Carmel, who has visited the area every winter since 2008 in the hope of a big find, has named the stone Amazing Grace, after her granddaughter.Miners Heritage, Rubyvale, home to Australia's largest walk-in underground mine, said: "We often get asked if there is still big sapphires found on the fields. "Had the pleasure of seeing first hand this beauty, specked at one of the fossicking areas. 424.55ct, about the size of a duck egg, predominantly yellow and named Amazing Grace."Pic courtesy Miners Heritage, Rubyvale