A gem trader said the stone was found by workmen digging a well in his home in the gem-rich Ratnapura area.
Experts say the stone, which is pale blue in colour, has an estimated value of up to $100 million in the international market.
The cluster weighs around 510 kilograms or 2.5 million carats and has been named the "Serendipity Sapphire".
"The person who was digging the well alerted us about some rare stones. Later we stumbled upon this huge specimen," Mr Gamage, the owner of the stone, told the BBC.
