HONG KONG — A white diamond the size of a small egg sold for $30.6 million at a Hong Kong auction Monday night.

But a blue diamond that was the night’s other highlight with a $19 million estimate failed to sell.

Two phone bidders competed for the 118-carat white diamond from Africa in six minutes of measured bidding until one dropped out in the Sotheby’s jewelry auction, part of fall sales of art and collectibles by the firm and its rivals.

The twice-yearly ritual in the southern Chinese city draws wealthy collectors from mainland China and other Asian countries and has made the city one of the world’s busiest auction hubs.