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 gems in the Crown Jewels of the United Kingdom, with a history dating back to the mid-14th century. According to a legend originating from the 1760s, the stone has been in the possession of the English and later British monarchy since it was given in 1367 by the Spanish king Peter of Castile to the then Prince of Wales, Edward of Woodstock, known as the "Black Prince". Henry V is said to have worn it at the Battle of Agincourt in 1415. The stone is believed to have been mined in Badakhshan in present day Afghanistan, the principal source of large spinel gems in the Middle Ages.
