NEW YORK—The American Museum of Natural History’s latest discovery is a diamond in the rough: a visitor’s $15,000 gem in a vacuum-cleaner bag.

The one-carat stone fell out of Catherine Hart’s ring during a “Night at the Museum” sleepover event at the storied New York institution Dec. 13 and 14.

Custodial supervisor Herbert Andujar says staffers in gloves and masks combed through the dust in four vacuum bags, scrutinizing everything that glittered, before finding the gem.

Hart says she’s thrilled to have recovered the diamond, which her husband gave her in 2000. The 59-year-old intends to give it to her granddaughter someday.