When a jewel makes the cover of an auction catalog, you know the house has high hopes for its sale. It’s easy to see why this century-old ring earned such prime placement—and a $458,500 price—at Bonhams’ Oct. 15 Fine Jewelry ­auction. The 6.98 ct. D-color diamond, explains Virginia Salem, director of Bonhams fine ­jewelry department in New York, “was really a marquise, but the older marquises were sometimes rounded at the tips so they have a marquise-oval look. It was a really rare and unique stone for that reason.” The piece (original estimate: $250,000–$300,000) had belonged to an oil heiress, and arrived with “about 80 other items” from a bank vault. “Underneath, it had a beautiful gallery, which was diamond-set in platinum. It was a very light, feminine ­mounting,” she says of the Belle Époque–era details. “But the true value is the diamond. The stone tr