The banker's wide collection contained some 700 gems before his death in 1939, including the famous Hope Diamond, now at the National Museum of Natural History in Washington, D.C.
The spinel hasn't been available for 98 years, according to the auction house, and will be offered during the Bonham’s London Fine Jeweler sale later this month for an estimated $240,000 to $310,000.
"You just don't see pieces of this quality and provenance on the open market very often. It's very exciting," Emily Barber, a representative for Bonham’s, said in a statement.
Spinels have similar refractive qualities to diamonds and garnets, according to the Gemological Institute of America, and they're a fairly common occurrence in nature. Forbes noted that they're often mistaken for rubies.
