A huge, internally flawless diamond from India's fabled Golconda mines was sold at auction in Geneva on Tuesday night for 20.355 million Swiss francs ($20.5 million), Christie's said.

The rare, colourless stone weighing 76.02 carats once belonged to Archduke Joseph August of Austria (1872-1962), a prince of the Hungarian line of the Habsburgs.

Its pre-sale estimate was 15 million Swiss francs to 25 million Swiss francs.

The diamond was the star lot at Christie's semi-annual jewellery sale in Geneva, where more than 300 lots were on the block.