The seller told her it was a rough brown diamond. Runsdorf, in disbelief, said she wanted it verified by the Gemological Institute of America. Sure enough, it came back as a certified 65-carat ...

In the '80s and '90s, gem-quality brown diamonds were worth about $1,500 per carat, but now many sell for up to $10,000 per carat. And the price of larger stones has risen by as much as 25 times.

Many diamonds actually show very pale colours, most often in shades of yellow, brown and grey. They are rejected as gemstones and used in industry instead, often as abrasives. However, on the rare ...