The Black Prince's Ruby is a 170-carat gemstone in the British Imperial State Crown. Despite its name, it is a red spinel from Badakhshan, not a ruby. Its bloody legend involves 14th-century Spain, Don Pedro the Cruel, and Edward the Black Prince. Beyond this famous stone, modern gemology uses advanced tools like spectroscopy and mass spectrometry to distinguish rubies from spinels, detect synthetic treatments, and trace a gemstone's geographic origin.