When Ryan Strasser, then an associate at Hogan Lovells in Washington, D.C., got down on one knee and asked Sarah Jones Dickens to marry him, he knew one thing: She’d like the ring. It was 4.06 carats, a near-colorless, old European cut brilliant diamond, mounted in platinum with 14 diamond accents. And it fit what he says were her must-have requirements: 3.5 to 5 carats with an inclusion rating of no “worse” than VS2 and a color rating of no “worse” than G, and with no fluorescence. The price? $100,000. And then, 11 months later…they broke up. According to Strasser,...