Black Prince's Ruby

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 at the front of the Imperial State Crown. 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 "Black Prince"). In 1820, the gemstone was valued at £10,000 (equivalent to £720,885 as of 2015).

All red gemstones used to be referred to as rubies or "balas rubies". It wasn't until 1783 that spinels were differentiated from rubies. The two gemstones can be distinguished on the basis of their chemical properties: a red spinel is a compound of magnesia, iron and chromium, while a ruby is a type of aluminium oxide. The rarity of this spinel, however, is that it is the biggest uncut spinel in the world, given that it has only been polished slightly, and has never received a proper cut, gemologically speaking.

It is possible the gem originates from the historic ruby mines in present-day Tajikistan.

The Black Prince's Ruby enters the "stage of history" in middle of the 14th century as the possession of Abū Sa'īd, the Moorish Prince of Granada. At that time, the rule of Castile was being centralized to Seville and the Moorish Kingdom of Granada was being systematically attacked and reverted to Castilian rule as a part of the Christian Reconquest of the Iberian peninsula. Abū Sa'īd in particular was confronted by the belligerency of nascent Castile under the rule of Peter of Castile, also known to history as Don Pedro the Cruel. According to historical accounts, Abū Sa'īd wished to surrender to Don Pedro, but the conditions he offered were unclear. What is clear is that Don Pedro welcomed his coming to Seville. It is recorded that he greatly desired Abū Sa'īd's wealth. When Abū Sa'īd met with Don Pedro, the don had Abū Saī'd's servants killed and may have personally stabbed Sa'īd to death himself. Upon searching Sa'īd's corpse, the spinel was found and added to Don Pedro's possessions.

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