The Black Prince's Ruby is a large, irregular cabochon red spinel weighing 170 carats (34 g) set in the cross pattée above the Cullinan II diamond at the front of the Imperial State Crown of the United Kingdom. The spinel is one of the oldest parts of the Crown Jewels of the United Kingdom, with a history dating back to the middle of the 14th century. It has been in the possession of England's rulers since it was given in 1367 to its namesake, Edward of Woodstock. The stone is believed to have originated from the Badakhshan mines in present day Afghanistan and Tajikistan, the principal source of large spinel gems in the Middle Ages.

Spinel is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula MgAl2O4 in the cubic crystal system. Its name comes from the Latin word spinella, a diminutive form of spine, in reference to its pointed crystals.