Jewelry designer Nina Runsdorf was at a trade show in the early 2000s when a “large brown rock” the size of a pebble caught her eye. The seller told her it was a rough brown diamond.

To get a closer look, we visited the jewelry company Suna Bros of NYC. More from So Expensive Narrator: Rihanna wore a $130,000 chocolate diamond necklace to the 2018 Grammys. And these Chocolate ...