Overheard at a clothing store in Brooklyn Heights last month: two women, about 30 years old, talking about rings, including a breathless anecdote about a mutual friend who’d just received a lovely style “that wasn’t too flashy.”The act of giving or receiving a ring to commemorate a betrothal is so commonplace in our culture that a conversation about it borders on the banal—unless you’re a jeweler or a devoted spectator of the biz, like I am, in which case any off-the-cuff comments about rings should be treated as wholly unbidden and, thus, incredibly valuable market research.Here’s another example: A good friend in Los Angeles recently asked me to help him find an engagement ring for his longtime girlfriend. “Can we do something more interesting than a diamond, or do I have to line up with the rest of the suckers?” he asked.I recently visited designer Paolo Costagli’s