Dear Tom,What are mother-of-pearl clouds?Oscar Johns, WilmetteDear Oscar,They are polar stratospheric clouds, also known as nacreous clouds or simply pearl clouds. They are rare, high altitude clou… ...

So products containing mother of pearl–like these brightly colored, bling-y watches on the block at Christie’s Oct. 20–must be sold at auction with a mouthful of a warning.

Mother of pearl is the coating found on the outside of pearls, and the inside of certain mollusc’s shells. It's one of nature's hardest, stiffest, most stable materials – and scientists have ...