A 10.33-carat Burmese ruby sold for an impressive $5.5 million after a six-minute bidding war among two bidders on the phone and one in the New York saleroom at Sotheby’s Magnificent Jewels auction.

The legendary “pigeon’s blood” ruby is poised to make a giant splash at Christie’s this fall. Two pieces of high jewelry featuring the exceedingly rare red gemstone will go under the gavel at the Hong ...