Spinel gemstones. Sri Lanka (8.35, 9.20, 9.30, 15.22 // 4.78, 11.23, 7.27, 5.46, 3.96 // 11.98, 8.53, 7.98, 14.96). Photo ? Joel E. Arem, PhD, FGA. Used with permission.
Although spinel has been used in jewelry since ancient times, this gemstone has only recently received the attention it deserves. Before the rise of modern gemology in the?late 19th and early 20th centuries, spinel was often identified as corundum, as they are often found in the same mines.?Nevertheless, these minerals are chemically distinct.?Subsequently, as jewelry enthusiasts were told some of their treasured rubies and sapphires were actually spinels, the stone’s reputation suffered. Furthermore, synthetic spinel is inexpensive and common. It has frequently been used as a simulant for other gems in class rings and birthstone jewelry, which has affected?its public perception. Natural spinel, however, has always been a rare and beautiful gem. As more information comes to light about the extensive and invasive enhancements lower-grade ruby and sapphire receive to “pump up” their color or clarity, educated consumers have come to appreciate spinel’s natural beauty. The natural spinels in today’s market are almost all untreated. Their relatively modest prices, availability in nearly any color, hardness, and suitability for most types of jewelry make them even more inviting.
Reds and pinks: crimson in LW, also SW; red in X-rays; no phosphorescence. Blue: inert in UV. Deep purple: red in LW, essentially inert SW, lilac in X-rays. Pale blue and violet: green in LW, X-rays, essentially inert in SW. Orange, red and pink; inert to weak red or red/orange SW. Weak to strong red and orange LW. Cobalt blue; strong chalky whitish green SW. Inert to moderate orange or orange/red LW. Near colorless and light green; inert to moderate orange/red LW. Deep purple; red LW, inert SW. Pale blue and violet; green LW, inert SW.
Fluorescent, UV-Long, UV-Short, X-ray Colors
Natural spinels usually not enhanced, but may receive heat treatment. Synthetic spinels may be quench crackled.
Red and pink; Strong fluorescence between 490 and 595 nm, weak band at 656, sharp lines at 685.5 and 684 nm. May also show chromium spectrum, broad band at 540 nm and absorption of violet. Blue; Strong band at 458 nm, narrow at 478 nm, weak lines at 443 and 433, may also have bands around 430 to 435, 550, 565 to 575, 590 and 625. Violet and purple; May show same spectrum as blue, only weaker.
Possibly from the Latin spina for “thorn,” alluding to spine-shaped crystals. Since this is not a common habit for spinel, this origin is uncertain.
Spinels are found in metamorphic rocks and their weathering products. Especially found in contact deposits (marbles and limestones).
See "Identifying Characteristics" below.
Spinel gemstones. Myanmar, (5.30, 2.98, 3.07 // 2.34, 10.98, 3.95 // 8.89, 2.68, 3.21. Photo ? Joel E. Arem, PhD, FGA. Used with permission.
Spinels range from strong, deep colors to very light pastels, coming in all shades of pink, lavender, red, red-orange, purple, blue, and even black. (Only pure green and yellow appear to be absent). These gems are found on every continent. The varieties are usually referred to by their color. The red and blue varieties are perhaps the most celebrated since they were historically identified as rubies and sapphires. (Spinels are also close to corundum in terms of luster and hardness).
Spinel is actually a large group of minerals. Gahnite, hercynite, ceylonite, picotite, and galaxite are all part of the spinel group. These materials are very dark and rarely used as gems. They’re all isometric oxides of magnesium, iron, and zinc, with traces of aluminum and other elements. There is a solid-state solution between spinel (MgAl2O4) and gahnite (ZnAl2O4). The intermediate species is known as gahnospinel ([Mg, Zn]Al2O4). Unlike most solid-state series, like garnet, pure spinel is much more common than its blends.
Spinel, Gahnite. Brazil (1.56). Photo ? Joel E. Arem, PhD, FGA. Used with permission.
Spinels are allochromatic gemstones. This means that when the mineral is pure, it’s colorless. Colors are derived only from the presence of trace elements acting as chromophores. Chromium, iron, and cobalt are the most common chromophores in spinel. The majority of jewelry gems, such as beryl (emerald, aquamarine, etc), corundum (ruby and sapphire), quartz, and topaz, are allochromatic. All have colorless varieties formed without the color-making impurities. In most of these cases, the colorless varieties are the most common and therefore the least valuable. Spinel is an exception. Until quite recently, no colorless specimens had been found in nature. Although colorless material could be synthesized by the bucketful in labs, apparently the conditions under which this gem forms in nature rarely exclude the coloring trace elements, making the colorless natural stone a rare and valuable collector’s gem.
Star stone spinels with 4-rayed stars have occasionally been cut from gray or grayish blue to black stones from Myanmar. A 6-rayed star can be seen in such material if oriented along the 3-fold symmetry axis of the crystal (parallel to the edges of an octahedral face).
Alexandrite-like spinels are known that are grayish blue when viewed in daylight and amethystine color in incandescent light. These are quite rare and usually small. Some stones from Sri Lanka change from violet (daylight) to reddish violet, due to the presence of Fe, Cr, and V.
Regardless of color, spinel’s high refractive index ensures excellent brilliance in a well cut and polished stone. Its hardness of 8 makes it a good choice for almost all jewelry applications, including rings. However, a high “Tiffany-type” setting for an engagement ring or a signature ring for 24/7 wear is not advisable.
The Black Prince’s Ruby is the large, red cabochon at the front of the Imperial State Crown of the United Kingdom. Before the advent of gemology as a science, stones like this were called “balas rubies.” In fact, the gem is a spinel. Other well-known historic gems long considered corundum, including the Timur Ruby, another crown jewel of the UK, have turned out to be spinels. “Imperial State Crown,” Great Britain, Tower of London. Public Domain.
Very distinctive spectra, useful in identification.
Red and pink: chromium spectrum, has broad band at 540 nm, plus absorption of violet. Group of fine lines in the red may be fluorescent “organpipe” lines.
Blue: iron spectrum has lines in blue especially at 458, plus narrow line at 478 and weak lines at 443 and 433. Two strongest are at 686, 675, plus 635, 585, 555, and 508. (Note: This iron spectrum is distinctive vs. the cobalt blue of synthetic spinel.) Nigerian blue gahnite also has bands at 700 and 570 like those seen in spinel.
Mauve and pale blue: similar spectrum to blue, but weaker.
Spinels are generally free of inclusions, but some inclusions are distinctive. Silk, as in sapphires and ruby, is seldom seen in spinel. Angular inclusions known as spangles are seen. Rows and swirls of tiny octahedra of another spinel, such as magnetite (Fe3O4), are distinctive.
Octahedron-shaped inclusions are the most common feature of spinels from all sources. In some cases they may actually be solid crystals of spinel. In other cases, they may be seen as fingerprints with tiny rows of octahedra-shaped negative crystals. In certain cases, these negative crystals may be filled with a foreign substance (such as white dolomite, yellow phlogopite, or black hercynite spinel). “Natural Spinel, Myanmar; Mogok/Namya.” Photo by E. Billie Hughes. ? Lotus Gemology. Used with permission.
Iron-stained films and feathers, especially at edges of gems, zircon inclusions and darkened surrounding areas, and zircon haloes (due to radioactivity) accompanied by feather around zircon due to stress cracking are also characteristic.
Natural spinels also contain octahedron-shaped cavities (negative crystals), sometimes filled with calcite.
Specimens from Mogok, Myanmar may contain inclusions of calcite, apatite, dolomite, and olivine. ?Specimens from Sri Lanka may contain inclusions of zircon, sphene, baddeleyite, phlogopite, apatite, and spinel.
The flame fusion process invented by Auguste Verneuil in the late 19th century for making synthetic sapphire can also be used to synthesize spinel.
Colorless spinels are most likely synthetic. (Aside from the very rare colorless natural spinels, there are also very light, almost colorless natural specimens). These colorless synthetics make nice diamond simulants.
Remarkable natural blue stones containing cobalt may be difficult to distinguish from flux-grown or flame fusion synthetics. However, ?ame fusion synthetics often display chalky, whitish green fluorescence in SW UV and strong red in LW. These synthetics also display “crosshatched” or “snakelike” anomalous birefringent patterns in cross-polarized light. Natural cobaltian spinel also shows absorption bands at 434, 460, and 480 nm, which are not observed in synthetic material. The band at 460 is especially diagnostic.
Natural spinels typically are not enhanced?but may receive heat treatment. Synthetic spinels may be quench crackled to simulate natural fractures.
Found traditionally and mainly in Afghanistan, Myanmar, and Sri Lanka, spinels have been discovered more recently in?various sites in Africa, Australia, Russia, and Vietnam. These gems?are usually not mined from the hard rock primary deposits in which they form but instead from alluvial or placer deposits where eroded material has been washed downstream. These “gem gravels” may contain other species in addition to spinel. The rough and tumble action has smoothed these crystals into rounded shapes and removed much of the included and fractured parts of the gem rough. This alluvial rough is?easily recovered material that’s great?for faceting.
Mining methods range from low technology (one or more miners with straw baskets sluicing the stream gravel) to the use of backhoes and hydraulic hoses for removing overburden from long-buried stream beds and supplying water for gem separation by a small crew.
Alluvial spinels and rubies, mined in Vietnam. Image courtesy of Gems from Earth.
Afghanistan: fine red spinel, source of many large gems of the ancient world.
Myanmar: from gem gravels, often found as perfect octahedra.
Sri Lanka: worn pebbles in wide variety of colors, especially pinks and blues; all the blue ones have a trace of Zn; many from Sri Lanka are black. The rare cobaltian variety is unique to Sri Lanka.
Jemaa, Nigeria: fine blue gahnite, SG = 4.40-4.59, RI = 1.793.
Madagascar: blue, gemmy gahnite.
Australia, Sweden, New Zealand: gahnite.
Russia: gemmy, fine pink material from Kuchi Lal in the Pamir Mountains.
Cambodia, Thailand, Vietnam: from alluvial gravels
United States: California; Colorado; Massachusetts; Montana; New Jersey; New York; Virginia.
The many colors of spinel: a suite of African spinels in purple, blue and pink, a top red Myanmar gem, an African lavender stone, a pink specimen from Russia, a “padparadscha” colored African piece, and an opaque black stone. Photo courtesy of Barbara Smigel, Artistic Colored Stones.
Spinels are known up to hundreds of carats in various colors.
Smithsonian Institution, Washington, DC: 45.8 (pale purple, Sri Lanka); 36.1 (indigo blue, Myanmar) 34 (red, Myanmar); 29.7 (pink-violet, Sri Lanka).
British Museum of Natural History, London England: deformed red octahedron from Sri Lanka, 520; another crystal, 355.
American Museum of Natural History, New York: 71.5 (red, Sri Lanka).
Crown Jewels of England: “Black Prince’s Ruby,” red spinel, estimated at 170; “Timur Ruby,” red spinel, 361.
Diamond Fund, Moscow, Russia: fine red gem, over 400.
Banque Markazi, Teheran, Iran: red stone over 500, another over 200, one about 225.
In ancient times, red spinels were called “balas rubies,” likely after Balascia, the ancient name of a region now divided between Afghanistan and Tajikistan. This was an important source of these gems.
Although spinels were identified as rubies and sapphires centuries ago, these are now recognized as distinct gemstones. Any references encountered nowadays to “spinel ruby” or “spinel sapphire” are erroneous. Red or green garnets are sometimes misleadingly called “Arizona spinels.” Almandines are sometimes evocatively but wrongly called “candy spinels.”