Via the New York Times:

“The Hope Diamond’s 45.52 sparkling, steely blue carats make it the most famous diamond in the world — shrouded in mystery and intrigue since it was pulled out of the ground in 17th-century India…

…  Scientists already knew that natural blue diamonds had a smattering of boron. It gives them their color, and other unusual properties: The Hope, for example, glows orange-red when irradiated with ultraviolet light….

…They placed 78 blue diamonds, including the Hope, in an apparatus that fired gallium ions, which peeled off atoms from a patch about one five-hundredth of an inch wide. These exfoliated atoms — hundreds of millions of them — were then sorted by weight to reveal how many were boron….”

To learn the answer to the question in the title, read on!

 

Photograph: The Hope Diamond on display at the National Museum of Natural History in Washington, D.C., photographer – David Bjorgen,  Permission by Multi-license with GFDL and Creative Commons CC-BY-SA 2, see Wikipedia.