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'Hololith' ring made from a single sapphire.

Rock of Ages - Emperor Caligula's ring leads stellar exhibition at Royal Jewellers Wartski.

A precious jewel, created for the most infamous of Rome's Emperor's nearly 2000 years ago, is causing gem collectors from around the world to flock to London as it goes on sale tomorrow.

The exquisite solid sapphire hololith ring, a former star of the legendary collection of the 4th Duke of Marlborough in the 18th century, is thought to be valued at close to £500,000.

The sky blue stone is etched with a portrait of his last wife, the notorious Caesonia, said to have been so beautiful that the depraved Emperor paraded her naked in front of his troops.

She was played by Dame Helen Mirren in the controversial 1979 movie Caligula.

The ring is the star attraction at a selling exhibition of over 100 engraved gems collected by Royal jewellers Wartski opening on Monday.

The exhibition has sparked worldwide interest, with collectors from as far afield as Japan queueing outside their premises days before to be first through the door.