Someone Bought a $400,000 Diamond Ring at Costco

Customers browse in the jewelry department at a Costco store in Miami, Florida.

Photographer: Scott McIntyre/Bloomberg

Look out, Tiffany -- Costco is coming for you.

Quarterly sales at the U.S. warehouse-club retailer got a boost from a customer who made a “significant diamond ring purchase, in the $400,000 range,” Costco Wholesale Corp. Chief Financial Officer Richard Galanti told analysts Thursday on a conference call.