spinella

The Medieval Latin spinella is a diminutive of spina (thorn, spine), named for the characteristic octahedral crystal form of spinel — a magnesium aluminum oxide, MgAl₂O₄ — whose sharp-pointed crystal faces resemble tiny thorns or spines when examined closely. The crystal habit of spinel (perfectly formed octahedra with sharp apex angles) was distinctive enough that the name stuck in the lapidary literature, and 'spinella' appears in medieval European gem texts referring to a red stone that was invariably classified as ruby. The Italian spinella became French spinelle and English spinel. The mineral itself is a magnesium aluminum oxide that occurs in a wide color range — red, pink, blue, purple, orange, green, black, and colorless — with the red variety, colored by chromium, being virtually indistinguishable from ruby (chromium-bearing corundum) to the naked eye. Both are hard, both are deep red, both occur in the same geological environments (metamorphic limestones and placer deposits in Burma, Sri Lanka, and Afghanistan), and both were called 'ruby' for approximately a thousand years of gem trade.

The history of the spinel-ruby confusion is one of the great stories in the history of gemology. The famous Balas ruby — named for Badakhshan (Balascia in medieval sources), the region of modern Afghanistan and Tajikistan where red spinels were mined — was the prestige red gem of the medieval Islamic and European worlds. 'Balas ruby' was not a ruby at all but a red spinel from the Kuh-i-Lal (Mountain of Rubies) mines of Badakhshan. The same mines supplied red spinels to the Central Asian courts, to the Delhi Sultanate, to the Mughal emperors, and through overland and maritime trade to the royal treasuries of Europe. When these stones appear in medieval European inventories and chronicles as 'rubies' or 'balasses,' modern gemological analysis often reveals them to be spinels — a discovery that has revised the gem histories of several major crown jewels.

The most dramatic example of the spinel-ruby confusion is the Black Prince's Ruby — a large, polished, irregular red stone set in the front of the Imperial State Crown of the United Kingdom, above the Cullinan II diamond. The stone weighs approximately 170 carats and was presented to Edward of Woodstock (the Black Prince) in 1367 by Pedro the Cruel of Castile, who had acquired it from the Moorish King of Granada who had in turn received it from a murdered Moorish prince. The stone passed through Henry V, who wore it at the Battle of Agincourt in 1415, then through the Crown Jewels to the present day. When spectroscopic analysis was applied to the stone in the 19th century, it was revealed to be a spinel — a fact that does not diminish the stone's historical significance but entirely changes its mineralogical identity. It is now displayed at the Tower of London with the correct identification.

The systematic distinction between spinel and ruby was not possible until the development of systematic mineralogy in the late 18th and early 19th centuries. Jean-Baptiste Louis Romé de l'Isle, the French crystallographer, was among the first to clearly distinguish spinel from corundum (ruby and sapphire) on the basis of crystal form and specific gravity in the 1780s. Once the distinction was established and communicated through the mineralogical literature, gem dealers had to confront the fact that many of their most valuable 'rubies' were in fact spinels — a revelation that reshaped valuation practices and provoked considerable anxiety in the gem trade. Today spinel is properly recognized as a gemstone of independent value, with fine Burma red spinels sometimes commanding prices comparable to rubies of equivalent size.