A unique pink Argyle and white diamond ring valued at $577,000 has been stolen from an auction centre in central Sydney.

New South Wales Police said an employee discovered the ring was taken from a display counter at the centre in Ralph Street in Alexandria on Saturday, between 12.30pm and 1.25pm.

About 100 people were inside the centre around the time the ring was stolen, police said.

Each of the diamonds in the ring is laser-inscribed with a serial number.

Pink Argyle diamonds are rare gems sourced from the Argyle Diamond Mine in the East Kimberley region in Western Australia, which produces 90 per cent of the world's pink diamonds.

A 2.51-carat, fancy deep-pink gem from the mine attracted a record price for an Argyle diamond of more than $2 million in October 2013.

Police have encouraged anyone with information on the theft to contact Crime Stoppers.