Spinel – in depth
Since beauty, rarity and durability measure the value of a mineral as a gemstone, certainly red, light red and orange spinel (pronounced spin-ELL, not SPINE-el) should rank near the top. This, unfortunately, is not the case; in fact, not only the public but many jewelers are completely unfamiliar with this species. The reasons for this state of affairs are not always easy to discover. However, the best red colors have long been confused with rubies. Since spinel is associated with ruby in most all of the areas of the world where it is mined, this confusion is understandable. In fact, although the habit of spinel (which is a cubic mineral) is the octahedron, these crystals are frequently irregularly developed, so that they resemble the flattened prisms and rhombohedra of ruby to the miners. Then, too, the names used for red spinel, such as “Balas ruby” (possibly from Balascia, an old name for Badakshan in Afghanistan, or from Balaksh, a region in Ceylon ) and “spinel ruby” fostered confusion. The layman may be misled to believe that these terms are merely locality names for ruby, such as Burma ruby or Ceylon ruby. They are not recommended as descriptive terms for spinel.
The other colors in which spinel occurs tend to be subdued, or “grayed out”. Therefore, the blue and violet stones are seldom seen in jewelry. Colors that are sometimes mentioned for spinel, yellow, green and colorless, are so rare that they have no importance and are of interest only to collectors.
One very good reason that spinel is relatively little known is its rarity. Today, a fine red spinel of more than five carats in weight is uncommon. This was not always true, since some of the most famous gem-stones in history are spinels of great size, and some of the spinels in gem collections weigh more than 100 carats.
One of the most fascinating gemstones in all the world, because of its rich history and the events engraved upon it, is a large spinel called the “Timur Ruby”. Since 1612, this famous stone, called “Tribute of the World” in the East, has belonged to the same owners as the incomparable Kohinoor diamond. From the inscriptions still on the stone, we learn that this is the “ruby” that fell into the rapacious hands of the Tartar conqueror. Timur-i-Leng (the lame Timur), or Tamerlaine, in Western countries, when the conquered Delhi in 1398. Where the stone was mined will never be known, although Afghanistan has been suggested. It is rather dark red and does not have facets, only the natural faces polished; hence, it is almost without brilliancy. It finally came into the hands of the British East India Company in 1849 and was exhibited with several smaller spinels in a necklace in the Great Exposition of 1851. The stone weighs 361 carats. It was given to Queen Victoria and has remained in the Indian Room at Buckingham Palace, rather than with the Crown Jewels in the Tower of London; because of this, few persons have seen the stone and there are no known pictures of it. Just when the “Timur Ruby” was recognized as a spinel is uncertain. It was listed a, a “very large spinel ruby” in the official catalog in 1851, and the fact that it was set with three other large spinels indicates that the former Sikh owners appreciated its true nature.
A red spinel of more than 400 carats in weight and cut in Oriental style forms the apex of the crown made for Empress Catherine II of Russia in 1762. It forms part of the immense Diamond Treasurer of the USSR in the Kremlin. Large spinels are in the mineral collection of the British Museum of Natural History, including a polished stone of 520 carats taken during the war with China in 1861. The largest red spinel on display in the United States is a faceted stone of 71.15 carats in the American Museum of Natural History.
It is of interest to note that no famous rubies are mentioned in the British Regalia or in any other crown jewels. The number of large rubies is strictly limited, and none has the long and romantic history associated with any one of the famous red spinels. True, few of these large and famous spinels have an acceptable ruby color, but perhaps they were mistaken for rubies centuries ago. However, spinel was recognized as a separate species at least as early as 1587 in Burma, when the Burmese kings appointed officials and issued titles such as “Lords of the Mines of Rubies, Sapphires and Spinels.” Possibly the rubies recovered in those days were held in such esteem by the local rulers that they were not allowed to leave the country. Since the spinels were not valued as highly, they may have been released in order to satisfy foreign demand. Surely, the lapidaries of the period must have recognized the greater hardness of the ruby.
It is difficult to be sure of the derivation of the name spinel. It may be from the Latin “spinella”, the diminutive of “spina” ( a thorn), from which we get our word spine. This word does not describe spinel crystals very well, unless one thinks of a well-formed octahedron as a thorn. It has been suggested that the word spinel may be from a Greek word meaning “spark” in reference to the bright-red and bright-orange color of some natural crystals.
The Spinels that the jeweler is most likely to see in the trade are the magnesium-aluminum spinels; that is, they are magnesium aluminates, with the chemical formula expressed as MgAl2O4. Since they are aluminium oxide, spinels are related to corundum (Al2O3) and to chrysoberyl (BeAl2O4). However, the magnesium and the aluminum may be replaced attiring formation by certain other elements without disturbing spinel’s cubic crystal structure, and we thus have different minerals. In this respect, spinel is like the garnet family of minerals and is considered a mineralogical group. Ordinarily, we do not recognize these other spinels in gemology, because, unlike the many species of garnet, these isomorphous spine relatives do not provide gemstones. One exception is the blue GAHNOSPINEL(GAH-no-spin-ell), in which part of the magnesium is replaced by zinc, producing a mineral halfway between ordinary gem spinel and the mineral GAHNITE (ZnAl2O4). In which the magnesium is completely replaced by zinc, resulting in a stone that is usually black and of no gem significance. In recent years, however, gahnite has been found in Brazil as dark tourmaline green colored crystals. Another exception is CEYLONITE (SELL-oh-nite), or PLEONAST (PIEE-oh-nast), in which part of the magnesium is replaced by iron, resulting in a black stone that is seldom seen in jewelry, except perhaps in calibre work. If iron replaces the magnesium and charomium replaces part of the aluminum, we have PICOTITE (PICK-oh-tite), a dark green to black spinel that occasionally occurs in Ceylon and is sometimes cut for collectors. One result of this isomorphous replacement in spinel is the difference in R.I. and S.G. the pure magnesium spinel always has the lowest values.
In the normal gem spinel, the presence of chromium, as a minor impurity accounts for the red and light-red colors, just as it does in ruby and pink sapphire. The violet colors are probably due to manganese impurities, and iron accounts for the blue colors.
It is interesting to note that synthetic spinel, which was discovered by Verneuil about 1915, is not made to reproduce the most popular color (red) of natural spinel, but in colors to imitate entirely different species. Red synthetic spinel has been produced only on a very limited basis in small sizes and only in recent years. Blue synthetic spinels are made to imitate aquamarine or sapphire, rather than the somber hues of natural spinel. Green stones made synthetically are used to imitate peridot and dark-green tourmaline. Pink stones do imitate natural pink spinel and sapphire, as well as pink topaz, pink beryl, kunzite and tourmaline. In fact, by sight alone, the pink synthetic spinel may be very deceiving.
Varieties And Trade Names of Spinel
Because spinel is so little known, it is the contention that the species should be described by color terms; instead of attempting to popularize variety names. However, in the following list, the old variety names are mentioned for purposes of assisting the student in recognizing them when they are encountered in older literature.
Red Spinel, Ruby-Colored Spinel, or Ruby Spinel
A misleading term, “spiel ruby” has been used for this variety. The best colors are those that approach the appearance of fine ruby, although very dark garnet reds to light purplish reds bordering on pink are included under this heading.
Purple, or Almardine, Spinel
Purple to red-purple.
Pink, or Rose, Spinel
Tones of light red to light purplish red. A misleading term for these colors that was formerly used is “Balas ruby”, although in older literature it was used for all red to light-red spinels.
Orange Spinel
This color was formerly called rubicelle (from the French “rubace”, a form of “rubis”, meaning “ruby”). Another name for the brightest of these yellow-orange to orange- red colors is flame spinel.
Blue Spinel, Sapphire-Colored Spinel, or Sapphire Spinel.
Rarely is this vaRarely is this variety a fine blue; more often it is a grayed-out dark blue to violet-blue or greenish blue. Even the best of these are collectors items, rather than commercial stones. Except by taking an R.I. or S.G., gahnospinel is indistinguishable from ordinary blue spinel. An older term, “sapphirine”, is misleading and should be discontinued.
Alexandrite like Spinel
This is a comparatively rare spinel in which the color is a light grayish blue in daylight and an amethystine violet under artificial light.
Black Spinel
This opaque spinel is rarely seen now that mourning jewelry is out of fashion, although it has appeared in some modern calibre work, such as guard rings.
Other colors of spinel would be called by the color involved. Thus, colorless and yellow spinels have been reported but not authenticated by the Institute. Green spinel, except in unattractive and nearly opaque stones, is rare and has been called CHIDRSPINEL, a term of little use to the practical gemologist.
Star spinels have been reported and described in gemological journals. However, a search of the literature seems to limit the number so reported to fewer than ten stones. The three stones seen by the Institute’s staff were imported from Ceylon as “black star spinel sapphires”. The stones were determined to be spinel, with rutile needles as the cause of the star. The rutile needles are oriented parallel to the edge of the regular octahedron. The inclusion causing the star in other spinels have been identified as acicular sphene crystals. Most of the stones have been described as having a six-rayed star on the apex of the cabochon, with alternating six and four-rayed stars at the girdles (see illustration). All but one of these stars have been described as dark purple to black in color. The exception was described as a gray stone from Burma.
Formation of Spinel
Almost all gem spinels have been formed by contact metamorphic activity associated with intrusions of molten rock Masses into impure limestones or dolomites. However, as with so many gemstones that come from Burma and Ceylon, the primary source is not mined, for it requires the concentration effected by nature in alluvium to make deposits rich enough to mine. Spinels of non gem quality occur in certain aluminum-rich basic igneous rocks, as well as in deposits that arise from the metamorphic alteration of these rocks.
The spinels from the countries mentioned above are associated with corundum and undoubtedly were formed in the same rocks as the rubies and sapphires. Since both spinel and corundum are durable minerals, they occur together in the gem gravels.
Sources of Spinel
In medieval times the source of fine spinel was the ruby mines in the Badakshan region of Afghanistan, where it was mined in alluvial deposit. Possibly this source accounts for the very large stones of hi-story and in collections and museums. Today, it cannot be considered a source of either ruby or spinel.
Burma
At present, Burma is undoubtedly the source of the best gem-quality spinel, just as it is known for the quality of its rubies. Here the spinel is mixed with the rubies in the gem gravels, although not in the same proportions in the various mines. Oddly enough, the rubies are usually badly waterworn, so that few crystal faces are exhibited, whereas many of the spinel crystals occur as excellent octahedral or twins that show only slight abrasion. No completely satisfactory explanation has been offered for this phenomenon, since corundum is appreciably harder (9) than spinel (8). In the Burma deposits, spinels of brilliant red, orange and pink, as well as the more somber blues and violets, are found. Fine blue stones have been reported, but they seldom leave the hands of local collectors, in whose collections travelers have reported seeing them.
From 1597 until 1887, when the Burma Ruby Mines, ltd., a British company, began production, the Burma deposits were mined in a primitive manner by licensed miners. More mechanization, drainage canals and other modern improvements were used by the British company. However, this firm suffered much adversity after the first few lucrative years, and the final closing in 1931 marked the end of its career. Since that time the area has been mined by natives in a manner that differs little from the primitive methods used 400 years ago.
Since the spinels recovered are, in effect, a by-product of the search for the more valuable corundum gems, ruby and sapphire, the mining methods outlined in that assignment apply equally to spinel.
Ceylon (Sri Lanka)
Spinel that occur in the gem gravels in Ceylon tend towards the blue and violet hues. However, good red and pink spinels are recovered occasionally that vie with any from Burma for quality. As the name suggests, black Ceylonite spinels are recovered here, as well as some very dark-green to brownish-green stones, which are rarely encountered in the market. In Ceylon, as in Burma, the spinels are recovered more or less as a by-product of the mining of gem gravels for the more valuable sapphires, star sapphires and the rare Ceylon rubies. The mining process is described in detail in both the corundum and the zircon assignments.
Cambodia and Thailand.
A few spinels are recovered in Thailand and the Pailin area, which borders on Cambodia. Here, as in other areas, the primary target of the miners is not spinel but the more valuable, transparent sapphires and the popular black-star sapphires. The deposits are alluvial, as they are in Burma and Ceylon. Spinels are also found in Thailand near Chantabun, an area renowned for its dark rubies and fine sapphires in alluvial gravels.
Other Sources
A few dark-blue spinel crystals have been recovered at the base-metal mines at Jemaa, Nigeria, Africa. Other localities are of mineralogical interest only, and have not provided the trade with any marketable stones.
Spinel Jewellery
Most spinels that reach the jewelry trade have been polished in their country of origin. Because color is the main consideration, the cuts employed are not usually precise; therefore, the fine colors that warrant the expense are usually re-cut in the United States. A characteristic of many commercially-cut spinels is their tendency to be too shallow, because many of the best crystals are flattened octahedral or twins. These twins are so characteristic of spinel that when they occur in other cubic minerals, such as diamond, they are sometimes called spinel twins by crystallographers. When the proportions are good, the brilliancy of the lighter colored stones is excellent. This brilliance is partly explained by the nature of the polished surface. Because of the heat developed during the polishing operation, most stones develop a liquid like layer on the polished surface, which, upon solidification, usually crystallizers. Spinel, however, is an example of a gemstone on which this layer remains amorphous and therefore smoother than a finely crystalline surface. This layer is called the “Beilby layer”, after Sir George Thomas Beilby, a British physicist.
Although most spinels worth cutting are faceted, the rare star spinel is always cut in cabochon. Flawed but attractively colored material is occasionally fashioned in the cabochon style or used for polished beads for necklaces or rosaries. Since spinel is not heat sensitive, only the usual care is required in the various operations.
No special orientation is required, unless it is necessary to eliminate or minimize flaws. Performing is accomplished on coarse and fine silicon-carbide grinding wheels. Grinding the facets may be done on a copper lap with diamond powder or on a lead lap with fine silicon-carbide powder. It polishes well on a tint lap with either tin oxide or Linde A powder as the polishing agent. A type-metal lap with Linde A also gives good results. Although polishing the facets seldom proves troublesome, a change of polishing direction will usually overcome any difficult that may be encountered. Suggested angles are 37° for the crown facets and 42° for the pavilion facets. Spinel lends itself well to the step or emerald cut; however, many are cut in the brilliant style.
Physical & Optical Properties of Spinel
Physical Properties |
|
| Chemical Composition | A double oxide of magnesium and aluminum, expressed by the formula MgAl2O4. |
| Crystallographic Character | Cubic system. Habit : Octahedra and twinned tones. Most gems are found as waterworn pebbles. |
| Hardness | 8 |
| Toughness | Good |
| Cleavage | Poorly developed. Cleavages parallel to octahedral faces may rarely occur, but they are too difficult to detract from toughness. |
| Fracture | Conchoidal |
| Specific Gravity | 3.57 to 3.90; normal 3.60; Gahnospinel 4.01; Ceylonite, 4.01; and gahnite 4.0 to 4.6. |
| Streak | White. Very dark stones may show a tinted streak. |
| Characteristic Inclusions | Minute octahedral crystal arranged either singly or in “fingerprint” patterns. Stones may be flawless, of course. |
Optical Properties |
|
| Degree of Transparency | Gem-quality material is transparent to semitransparent. Ceylonite and gahnite may be opaque. Star spinel is semi-translucent |
| Luster | Polished surfaces are subadamantine; fracture surfaces are vitreous. |
| Refractive Index | Gem material is usually 1.718 and almost always between 1.715 and 1.720, but very rarely as high as 1.735. Gahnospinel, 1.74 to 1.78; gahnite , 1.80. |
| Birefringence | None |
| Optic Character | Isotropic (singly refractive). |
| Pleochroism | None |
| Dispersion | 0.20 |
| Phenomena | Asterism encountered rarely; change of color; fluorescence |
| X-Ray, Fluorescence | Red and pink stones glow red; other colors are inert. |
| Ultraviolet Fluorescence | Red and pink stones glow red under long wavelength radiation in direct relation to depth of color, with the exception of dark-red stones, which glow weakly |
| Absorption Spectra | Red stones show sharp lines in the red at 6855 and 6840 A.U., a weak band at 6560, and a strong absorption in the yellow-green from about 5950 to 4900 A.U. In the pink and bright-red stories, the spectrum may show a series of five bright-red fluorescent lines in the red. The spectrum is caused by chromium.
Blue stones show a band of medium strength in the orange at 6320 A.U.; a somewhat stronger and narrower band at 5550; and a strong broad band at 4590, with a narrow line at 4800. More or less complete absorption occurs below 4300. Many stones show the bands in the yellow and orange so weakly as to be nonexistent. The spectrum is caused by ferrous iron. Other colors, such as violet and purple, may show the ferrous spectrum weakly, depending on the strength of the blue component of the color. Gahno spinel and gahnite tend to have the same spectrum as blue spinel, only much stronger. The 5920 line in the yellow and the 5550 line in the green may be quite distinct, whereas the 4590 band may be so broad as to over shadow the 4800 line. |
Effects Caused by: |
|
| Heat | Light colored stones may fade under intense heat. Spinel melts at approximately 2135° C. |
| Acids | Insoluble. |
| Irradiation | No data available. |