Blue diamonds account for less than one percent of all mined diamonds, and thus a fancy deep-blue one is considered an exceptional discovery—which is why Bonhams’ offering of a 5.30-carat deep-blue diamond ring is making news. The stone will be on the auction block at the firm’s April 24 Fine Jewelry sale in New York. The cushion-cut diamond sits in an elegantly simple circa-1965 Bulgari ring, surrounded by white diamonds. The ring is expected to fetch from $1.5 million to $2.2 million—though it has the potent…