(IDEX Online) - The world's largest sapphire cluster - weighing 2.5 million carats, or more than half a ton - has been found by workers digging a well at the home of a gem trader in Sri Lanka.The pale blue stone (pictured) has been provisionally valued at $100m, although the quality of the stones inside the cluster is not yet known."The person who was digging the well alerted us about some rare stones. Later we stumbled upon this huge specimen," the owner, Mr Gamage, told the BBC.The biggest single sapphire ever found is the 1,404-ct oval-shaped blue star sapphire, Star of Adam. It was found in Ratnapura - the city where the sapphire cluster was found, in 2015. The highest price ever paid for any sapphire at auction is $17,295,796 at Christie's Geneva in 2014 for the Blue Belle of Asia, a 392.52-ct faceted, cushion shape gem set in a necklace.