A Sri Lankan gem merchant claiming to have the world's biggest star blue sapphire is ready to sell the precious stone he says is worth at least $300 million.

The Gemological Institute of Colombo certified the gem as weighing 1404.49 carats. The institute's chief gemologist Ashan Amarasinghe says according to the available data, the stone could be the world's biggest blue star sapphire.

The oval-shaped gem is as large as a man's palm.

It was mined from a pit near the central town of Ratnapura that is famous as the "gem city" of Sri Lanka.

The present owner bought the sapphire last September.