The Black Prince's Ruby: The Gemstone That Isn't a Ruby

At the centre of the Imperial State Crown of the United Kingdom, set at the front of the lower band, just above the brim, is a polished red stone of approximately 170 carats. The stone is roughly the size of an egg. It is irregularly shaped, not faceted, with a small drilled hole near its top that has been plugged, sometime in the past two centuries, with a small genuine ruby. The crown sits in a vitrine in the Jewel House at the Tower of London. The stone has been in British royal possession, continuously or near-continuously, since 1367.

It is widely believed, including in much of the official Royal Collection Trust signage, to be a ruby.

It is not a ruby. It is a spinel.

What I find worth saying about the Black Prince's Ruby, which is the most direct way to phrase a sentence in which the noun and the description are in active contradiction, is that the misnomer is older than the science that proved it. By the time nineteenth-century gemmologists worked out that ruby (a corundum, chemically aluminium oxide coloured by chromium) and spinel (a magnesium aluminate, also coloured by chromium, but with a different crystal structure and a slightly lower hardness) were entirely distinct minerals, the stone in the front of the British crown had been called the Black Prince's Ruby for nearly five hundred years. There was, by then, no real possibility of renaming it. The name had become more historically substantial than the chemistry it incorrectly described.

The Crown made a choice, and the choice was to keep the name.

What follows is the history of a stone that has been worn by a Castilian king, a fourteenth-century English prince, a king at Agincourt, three Stuart monarchs after the Restoration of the British monarchy, every subsequent British sovereign up to the present, and that has spent its entire documented life carrying a description that is, technically, wrong.

Ruby, or spinel

Before the eighteenth century, no European gemmologist could reliably distinguish ruby from spinel. The two minerals look identical to the unaided eye. Both occur as transparent red crystals. Both are coloured by trace chromium. Both turn up in the same geological environments, often in the same alluvial deposits. The Mughal court jewellers, who handled both stones daily for centuries, had a category for "balas ruby," which we now know was the spinel; they treated it as a slightly less valuable variety of ruby rather than a distinct mineral, but they did know the two were not quite the same.

European jewellers, working with much smaller volumes of both stones, simply called them rubies. The most famous historical "rubies" of European royal collections (the Black Prince's Ruby, the Timur Ruby in the same Royal Collection, the Côte de Bretagne in the French Crown Jewels) are all spinels. None of them is, in modern chemical terms, a ruby.

The chemical distinction was clarified by the French mineralogist Jean-Baptiste Romé de l'Isle in the 1770s and confirmed through the nineteenth century as gemmological technique advanced. By the time the matter was settled, the major historical "rubies" had been called rubies for so long that the mineralogists themselves, while noting the correct identification, declined to lobby for renaming. The British Crown was not, in any case, in the habit of taking instruction from mineralogists.

Granada, 1362

The earliest reliable documentation of the stone places it in the possession of Abu Sa'id, the third Nasrid king of Granada, in the early 1360s. Granada at this point was the last surviving Muslim emirate in the Iberian peninsula, holding a substantially reduced territory in the south after centuries of the Reconquista, and Abu Sa'id had come to its throne through a violent succession in 1359. He was not a stable ruler. His reign lasted three years.

In 1362, threatened by both internal opposition and the kingdom of Castile to the north, Abu Sa'id sought safe passage and a negotiated settlement with Pedro I of Castile, known to history as Pedro the Cruel. Pedro granted the safe passage. Abu Sa'id arrived at Pedro's court in Seville with what was, by all contemporary accounts, an extraordinary amount of personal treasure. Among the treasure was the large red stone.

Pedro murdered Abu Sa'id, along with his retinue, and took the treasure. The murder was not significantly disguised. Abu Sa'id's body was reportedly thrown from a tower. The stone, henceforth, was Pedro's.

This is not the kind of provenance event that the modern Royal Collection Trust catalogue dwells on, but it is the foundational acquisition. The Black Prince's Ruby's documented history begins with an assassination.

Nájera, 1367

Five years later, Pedro the Cruel was himself in difficulty. He had been deposed by his illegitimate half-brother Henry of Trastámara, who had taken Castile with French military support, and Pedro was now an exile seeking foreign help to reclaim his throne. He approached the English, who were, at this period, the dominant military power in southern France and the natural opponents of any French-backed Castilian regime.

The English commander Pedro approached was Edward of Woodstock, the eldest son of King Edward III and heir to the English throne, known then and since as the Black Prince. The Black Prince was thirty-six years old, a veteran of Crécy and Poitiers, and the most successful European military commander of his generation. He agreed, against the advice of his own court, to help Pedro reclaim Castile. The English army crossed the Pyrenees in early 1367 and defeated Henry of Trastámara at the Battle of Nájera in April of that year.

The Black Prince's campaign was a tactical success and a strategic catastrophe. The Castilian terrain destroyed the English army through dysentery and exhaustion. Pedro refused, or was unable, to pay the promised costs of the campaign. The Black Prince returned to his French territories with his army broken and the war substantially unfunded.

Among the partial payments Pedro did manage to provide was the great red stone he had taken from Abu Sa'id five years earlier. The transfer is recorded in contemporary English chronicles. The stone was, from that point, English royal property. It travelled with the Black Prince back to Aquitaine and, after his death from illness in 1376, passed to his son, who became Richard II.

Agincourt, 1415

The most famous, and possibly least verifiable, episode in the stone's English period takes place at Agincourt on 25 October 1415. Henry V, the great-grand-nephew of the Black Prince by descent through John of Gaunt, was at this point wearing the stone as a centrepiece in his battle helm. The detail is recorded in a number of sources, though the earliest is from several decades after the battle, and the precise nature of the setting is unclear.

In the course of the fighting, according to the surviving accounts, the French Duke of Alençon led a charge against Henry's position and reached the king personally. Alençon struck Henry on the head with a war-axe. The blow dented Henry's helmet and brought him to his knees. Alençon was then killed by Henry's bodyguard. The helmet was severely damaged. The stone, by every account, was unharmed.

Whether the story is literally true is contested. The dented helmet itself exists, in the Royal Armouries collection at the Tower of London, and the damage is consistent with a heavy blow. Whether the Black Prince's Ruby was actually mounted in the helmet at the moment of the battle, or whether the story is a later addition to dramatise an already remarkable monarch, is the kind of historical question that does not admit of a definitive answer. What is clear is that the story was being told within Henry's own lifetime and that it became, by the Tudor period, a fixed element of English national mythology.

The stone, by this point, was a fixture of the English coronation regalia.

The Commonwealth break

The Crown Jewels of England were, by parliamentary order, dispersed in 1649 following the execution of Charles I. Most of the metalwork (the crowns, the orbs, the sceptres) was melted down for coinage. The stones were prised from their settings and sold separately to merchants and private collectors.

What happened to the Black Prince's Ruby specifically during the Commonwealth period is somewhat unclear. It is known to have been recorded in the parliamentary inventory of 1649 and valued at the relatively modest sum of £4 (a figure that reflected the marketplace's then-uncertain valuation of historical provenance, rather than the stone's intrinsic gemmological value). It is known to have been sold to a London jeweller during the Interregnum. It is known to have been reacquired by the Crown at the Restoration in 1660 or shortly afterwards, in time for the new coronation regalia commissioned for Charles II's coronation in April 1661.

The intervening route is the kind of provenance detail that historians of the Crown Jewels have spent careers attempting to clarify. The most plausible reconstruction is that the stone was sold cheaply during the Commonwealth, passed through several London dealers, and was identified as the historical Black Prince's Ruby in time for the Restoration. Whoever sold it back to the new Stuart court did well from the transaction. The stone has remained in the British Crown Jewels continuously from 1661 to the present.

The current crown

The Imperial State Crown in which the Black Prince's Ruby currently sits was made in 1937 for the coronation of George VI. It was not the first crown to hold the stone: a Victorian-era Imperial State Crown made for the 1838 coronation of Queen Victoria contained it, as did a Stuart-era crown after the Restoration. The 1937 version is a remake of the Victorian crown, with the same major stones reset into a new gold and silver frame, and is the crown worn at the coronation of Elizabeth II in 1953 and Charles III in 2023.

The Black Prince's Ruby sits at the front of the lower band, just above the brim, in the centre of the cross pattée. Below it sits the Cullinan II, a 317-carat cushion-cut diamond cut from the same rough as the larger Cullinan I (which is in the Sovereign's Sceptre). Behind, set into the cap, is the St Edward's Sapphire. The Stuart Sapphire is at the back. The crown holds, in total, 2,868 diamonds, 17 sapphires, 11 emeralds, 4 rubies, and 269 pearls. The Black Prince's Ruby, despite not being a ruby, is the unambiguous focal point.

The drilled hole in the stone, presumably from a much earlier setting in which it was worn as a pendant, has been plugged with a small genuine cabochon ruby. The contrast between the two stones (the great spinel and the small genuine ruby) is visible at close range. It is, more or less, the only place in the British Crown Jewels where the chemical reality of the spinel-versus-ruby distinction is publicly admitted, and it is hidden in plain sight.

On keeping the name

The decision to keep calling the Black Prince's Ruby a ruby, after nineteenth-century mineralogy had established that it wasn't one, was never, as far as I have been able to determine, formally taken. There is no Royal Warrant, no decree of the College of Arms, no parliamentary debate, on the question of whether to rename it. The name simply continued because the alternative would have required revising six hundred years of inscription, chronicle, royal record, and public reference. The name was older, and more substantial, than the science.

What I find quietly compelling about this, as a piece of cultural object lesson, is that the stone has effectively been allowed to remain what its history made it, even when the history was based on a misidentification. Most institutions, faced with the gap between what an object is and what it has been called, will correct the description. The Crown declined to. The Black Prince's Ruby remains the Black Prince's Ruby. The fact that it is mineralogically a spinel is offered, in the official guidebooks, as an interesting parenthetical, alongside the assassination, the battle, the Restoration, and the various royal heads that have worn it. None of these things are more or less true than the others. All of them, together, are what the stone is.

It is in the Jewel House. You can go and see it.

Frequently asked questions

Is the Black Prince's Ruby a real ruby?

No. The Black Prince's Ruby is a red spinel, not a ruby. The two stones look identical to the unaided eye, but ruby is a corundum (aluminium oxide) and spinel is a magnesium aluminate, with different crystal structures and slightly different hardness. The Black Prince's Ruby was identified as a spinel during the nineteenth century, but the historical name has been retained.

How big is the Black Prince's Ruby?

The Black Prince's Ruby is approximately 170 carats, making it one of the largest uncut red spinels in the world. It is irregularly shaped (polished rather than faceted) and roughly the size of a hen's egg. A small drilled hole near its top has been plugged with a small genuine ruby.

Where is the Black Prince's Ruby now?

The Black Prince's Ruby is set in the Imperial State Crown of the United Kingdom and is on display in the Jewel House at the Tower of London. The Imperial State Crown is the crown worn by the British monarch at the State Opening of Parliament and at the conclusion of the coronation ceremony.

Who did the Black Prince's Ruby originally belong to?

The earliest documented owner of the Black Prince's Ruby was Abu Sa'id, the third Nasrid king of Granada, in the early 1360s. After Abu Sa'id's murder at the hands of Pedro I of Castile in 1362, the stone passed to Pedro, and from Pedro to Edward of Woodstock, the Black Prince, in 1367 as partial payment for English military support at the Battle of Nájera. Earlier history of the stone before Abu Sa'id's possession is not documented.

Did Henry V wear the Black Prince's Ruby at Agincourt?

According to surviving accounts, Henry V wore the Black Prince's Ruby mounted in his battle helm at the Battle of Agincourt on 25 October 1415. The story holds that a blow from the French Duke of Alençon dented the helmet but left the stone intact. The earliest documentary sources for the episode date from decades after the battle, and the historical accuracy of the specific detail is contested, but the dented helmet itself survives in the Royal Armouries collection at the Tower of London.

What is the difference between ruby and spinel?

Ruby is a variety of the mineral corundum (aluminium oxide), with hardness 9 on the Mohs scale. Spinel is a magnesium aluminate, with hardness 7.5 to 8. Both are coloured red by trace chromium and appear identical to the unaided eye. The two minerals can only be reliably distinguished by gemmological testing including refractive index measurement and crystal structure analysis. Many historical "rubies" in European royal collections, including the Black Prince's Ruby and the Timur Ruby, are spinels.

Why is it called the Black Prince's Ruby?

The stone is named for Edward of Woodstock, the eldest son of King Edward III and heir to the English throne, who acquired it from Pedro I of Castile in 1367. Edward was known as the Black Prince, traditionally said to be due to his black armour. The "Ruby" in the name reflects the pre-eighteenth-century European inability to distinguish red spinel from ruby; the name has been retained despite the modern mineralogical identification because of its historical and cultural significance.