Spinel Compounds: Background & Historical Perspective
Submitted to the Journal of the American Ceramic Society, November, 1999
We are delighted to present this topical issue of the Journal of the American Ceramic Society, devoted to the structure and properties of spinel compounds. Though spinels have commanded the attention of both scientists and engineers in recent years (as illustrated by the papers herein), they did not always enjoy this favored status. For roughly nine-tenths of this past millennium, spinel slumbered in obscurity, eclipsed by seemingly more glamorous materials like corundum. In fact, a topical issue of the Journal of the American Ceramic Society focusing on corundum (alumina), predates this publication by several years (J. Amer. Ceram. Soc. 77 (2), Feb. 1994). To better appreciate spinel's relative insignificance, we need only consider the historical record regarding spinel.
Spinel (alternate spellings include spynel, spinell, spinal, espinal, and spinelle1) is an ancient name that originally referred to red gemstones, which today, we recognize as crystals of magnesium aluminum oxide, MgAl2O4. The origin of the name was lost in antiquity, though it may derive from the Latin word spina meaning "little thorn," a reference to the sharp points on some crystals.2 Spinels are documented in literature dating as early as the sixteenth century:
There is also… an other kynde of Rubies which wee caule Spinelle.
There is a great store of rubies, saphires, and spinelles in this Iland.
But in medieval times, spinels were known by other names, especially balas ruby or lal (lal is the Persian word for balas ruby; in Chinese it is la).5
The Spinel and the Balas, the one a lively poppy-red, the other a violet-rose, frequently usurp the dignity of a true ruby.
(Daily News, 1892)6
The name balas ruby apparently derives from an ancient word for Badakhshan, a province in the north of Afghanistan.7 Among the most storied stones of history are the large balas rubies found in museums and gem collections throughout the world:
…The majority of them [balas rubies] come from Afghanistan, from the mountains of the province of Badakshan. In old Russian manuscripts it was called 'lal Badakhshan.'
From the historical record, it is clear that the Badakhshan mines were of great importance during the period 1000–1900 AD. Even Marco Polo (ca. 1254–1324 AD) found pause to comment on the mines, as recorded in the following excerpt from Yule and Cordier's definitive version of Marco Polo's travels:9–11
BADASHAN is a Province inhabited by people who worship Mahommet, and have a peculiar language. It forms a very great kingdom, and the royalty is hereditary…
…It is in this province that those fine and valuable gems the Balas Rubies are found. They are got in certain rocks among the mountains, and in their search for them the people dig great caves underground, just as is done by miners for silver. There is but one special mountain that produces them, and it is called SYGHINAN. The stones are dug on the king's account, and no one else dares dig in that mountain on pain of forfeiture of life as well as goods; nor may one carry the stones out of the kingdom. But the king amasses them all, and sends them to other kings when he has tribute to render, or when he desires to offer a friendly present; and such only as he pleases he causes to be sold. Thus he acts in order to keep the Balas at a high value; for if he were to allow everybody to dig, they would extract so many that the world would be glutted with them, and they would cease to bear any value. Hence it is that he allows so few to be taken out, and is so strict in the matter.
It is safe to say that, based on numerous historical accounts, the Badakhshan mines were the source of many of the finest early rubies and red spinels in gem collections around the world, such as those in the crown jewels of Iran, the collection in Istanbul's Topkapi, Russia's Kremlin and Diamond Fund, and England's Tower of London.12 And of the most notorious titled rubies, most are not rubies at all, but red spinels. The most famous of all is found in the United Kingdom and is called the Black Prince's Ruby.
Few precious stones have such a long and storied history as the large, semi-polished, crimson orb known as the Black Prince's Ruby. The following account, reproduced from Hughes (1997),12 is based largely on Orpen (1890),13 Younghusband & Davenport (1919),14, and Sitwell (1953).15
Although the gem was probably mined at Badakshan's famous balas ruby mines along the Afghanistan border, the gem's first documented appearance is in fourteenth-century Spain. At that time, Spain was ruled by a number of petty kings, one of whom was a Moorish prince, Mohammed, of Granada. Don Pedro the Cruel ruled nearby Seville, and it was to him that Mohammed fled after being deposed by his brother-in-law, Abu Said. Don Pedro's army eventually brought Abu Said to heel. When they arrived to negotiate, Abu Said and his attendants were killed, and their jewels seized. The date was 1366. Among the jewels was a large red spinel octahedron, the size of an egg. It is today known as the Black Prince's Ruby.
Don Pedro soon found it his turn to flee, his adversary being none other than his own brother, Henry. In 1366, he fled to Bordeaux, where the Black Prince* kept court. Don Pedro beseeched the Black Prince to help, promising untold treasures in return. Henry was duly defeated and the large red stone passed as payment to the Black Prince, in 1367.
The gem reappeared in the hands of the English king, Henry V, at Agincourt, on Oct. 25, 1415. The gallant king, with his army reduced to 15,000 men, was falling back upon Calais when at Agincourt he encountered Duc d'Alençon, the French prince, and his army of 50,000 men. The morning of the climactic battle, Henry appeared dressed in most splendid attire, with gilt armor. Upon his helmet was a crown garnished with rubies, sapphires and pearls, including the Black Prince's Ruby.
Henry's helmet was more than mere decoration, for on that day he was set upon by the French prince, Duc d'Alençon. The Frenchman struck his helmet a mighty blow with his battle-axe, nearly killing Henry. Others also attacked him, even managing to break away a portion of the crown. Miraculously, though, both the stone and Henry survived.
From here, the precious gem passed through the hands of numerous British kings, including Henry VIII and his daughter, Elizabeth I, who kept it in her private collection. She did show it to a Scottish envoy, Sir James Melville, however. One evening the Queen took him into her bed-chamber, where "she shewed me a fair ruby, great like a racket-ball."
The Black Prince's Ruby now is mounted in the front of the Imperial State Crown, just above the famous Cullinan II Diamond. It is a huge, semi-polished octahedron.** Sitwell15 states that the stone is backed by a gold foil, as were many ancient gems, to improve its brilliance. This has not been removed for fear of damaging the gem. The stone measures some two inches (5.08 cm) in length and is of proportionate width.14 Its exact weight is unknown, but estimates put it at ~140 ct.
As the Black Prince's [Spinel] Ruby illustrates, with regard to spinels, there is no want of esthetic qualities and important properties, but there is lack of recognition. No better testimony to spinel's magnificent properties can be found, than to consider the magnetic characteristics of the ancient spinel mineral, magnetite (Fe3O4).
Magnetite is arguably the most important mineral, from a historical perspective, ever to be discovered.
Magnetite was known in ancient times as lodestone (or loadstone, or lodysshestone) which literally means "way stone,"*** from the use of the magnet in guiding mariners:1
One kept ye compas and watched ye our glasse, Some ye lodyssestone dyd seke.
(Unknown Author, 1515)18
Lodestones were used to magnetize the mariner's compass, the latter an instrument for determining the magnetic meridian and guiding a ship's course at sea:
Ane skyppar can nocht gyde his schip to ane gud hevin without direction of his compas.
The word lodestone was in fact, synonymous with magnet (or magnete):1
The Ilande of Magnete that is the Iland of the lode stone which is vnder or near abowte the northe pole.
Dinocrates began to make the arched roufe of the temple of Arsinoe all of Magnet or this Loadstone.
Quite a new epoch in the history of cartiography begins with the introduction of the magnetic needle into Europe. Hitherto the seaman had governed his course by the observation of the heavens; henceforth an instrument was placed in his hands which made him independent of the state of the sky. The property of the magnet or 'loadstone' to point to the north first became known in the eleventh century, and in the time of Alexander Neckam (1185) it was already poised on a pivot. It was, however, only after Flavio Gioja of Amalfi (1302) had attached to it a compass-card, exhibiting the direction of the winds, that it became of such immediate importance to the mariner.
But despite its exquisite properties, this miraculous stone has generally been eyed with puritanical suspicion:
Such things which are occasions and loade stones to draw people to wickednesse.
Finally, in the nineteenth century, the common structure of the many natural compounds of spinel came to be appreciated:
Chromate of iron… occurs crystallized in regular octahedrons, being… the analogue of magnetic oxides of iron, and the spinelle ruby.
Today, the list of compounds recognized to possess the spinel crystal structure is imposing. Table I (partly reproduced from Grimes24) illustrates the vast variety of spinels.
This topical issue is devoted to the structure and properties of some of these compounds. We hope that upon reading the following contributions, you'll be convinced of the merit due these special materials. And the next time you turn on your VCR, please pause to reflect on the splendid performance managed by the little maghemite (gamma-Fe2O3) spinel crystals inside your tape drive. Mind you that meanwhile, the hematite (alpha-Fe2O3) corundum crystals outside in your garage, are doing little more than adding rust to your car.
* The Black Prince
was Edward, Prince of Wales (1330–1376). His epithet "Black
Prince" may reflect the terror he inspired in the French, but
it probably referred to the color of his armor. [ return to
She was at home, abroad, in euery part, Loadstar and Loadstone to each eye and heart.
[ return to article ]
See also Rubies and Spinels of Afghanistan
In 1998, the American Ceramic Society held a special meeting in Cincinnati devoted to spinel and spinel-type compounds. Such compounds are of increasing interest to solid-state scientists, since one can play mix-and-match with ions, creating new properties while retaining others.
Kurt Sickafus of Los Alamos National Laboratory was aware that spinel was of interest not simply to scientists, but also had a long history as a gemstone and thought it might be fun to present another aspect of the subject to the conference attendees. Thus Edward Boehm of Joeb Enterprises and myself were invited to talk on the gem aspects of spinel. The above article was an outgrowth of that presentation.
In December, 1999, the Journal of the American Ceramic Society published an entire volume devoted to spinel compounds. This was the lead article.
Publication of this paper in what is the most rarified of scientific publications shows an openness to history and lore – to learning in general – which is unusual today. We applaud them for this. Now if we can only convince the editors of other publications to adopt a similar posture…
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