Spinel

Spinel blue spinel


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.

Spinels that the jeweler is most apt 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 spine is rarely This opaque spine 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 Jewelry

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.

Test and Identification of Spinel

By sight alone, it is possible to confuse spinel with other stones of similar color. However, aside from a rare pyrope garnet with a low R.I. and synthetic spinel, normal use of the refractometer and polariscope will identify spinel. With the rare pyrope, the use of either the spectroscope or ultraviolet light will separate them. All but the darkest red spinel will fluoresce under ultraviolet, whereas garnet never does. The absorption spectrum, particularly because of this fluorescence is different in the two minerals. The differences were explained in detail in Assignment of Spectroscope.

In general, any synthetic spinel with the color of a natural spinel will show an R.I. close to 1.73, rather than 1.72. In the polariscope, the synthetic, because of a slightly abnormal chemical composition and a consequent strained crystal structure, will show a patchy extinction. It may be necessary to hold a 10x loupe over the stone in order to see this clearly, especially in darker stones. Natural stones may show anomalous double refraction in the form of a very slight lightening and darkening when rotated in the polariscope, but not the patchiness of the synthetic. Natural spinel often seems entirety or very nearly devoid of the anomalous double refraction shown by most of the other singly refractive gem materials. The presence of octahedral inclusions proves natural origin; gas bubbles, either round or in long "gas hoses", proves synthetic origin. Gas bubbles in the synthetic are usually exceedingly small, and often appear to have a faintly octagonal outline. Usually, the colors of the synthetic are unlike those of the natural stones, being used, as mentioned previously, to imitate other gemstones, rather than natural spinels. Completely colorless, yellow, and marketable green spinels are almost certainly synthetic. A deep-red synthetic spinel is produced, but it is rarely seen on the market; its R.I. can be expected to be close to 1.721.

Richly colored blue substitutes for the Kashmir quality of sapphire are almost certain to be synthetic spinel, rather than synthetic sapphire. They are characterized by red overtones resembling strong red fire. These stones appear red through an emerald filter.

The possibility of glass having an R.I. similar to that of spinel exists. However, in order to bring the R.I. of glass to 1.72, a great deal of lead has to be added, which makes the S.G. much higher than that or spinel. Furthermore, it makes the stone too soft for jewelry use. Therefore, glass with an R.I. above 1.70 is a curiosity as a cut stone and is rarely encountered. Garnet and glass doublets are easily distinguished by magnification; they never have a garnet cap of an index in the gem spinel range.

A gahnospinel with an R.I. near that of sapphire does occasionally appear. The single refraction and lack of dichroism will separate it from sapphire, if the peculiar grayed-out color does not give it away by eye.

Valuation of Spinel

The most highly prized colors in spinel are undoubtedly the intense red to purplish red and the orange-red. Actually, many hues exist in the red to purplish-red category, varying from those that can be mistaken for pyrope or almandite to those that might be mistaken for ruby and those that are more nearly the color of pink sapphire. The connoisseur seldom confuses ruby and red spinel. The great clarity of this color of spinel, together with its lack of "dichroism and the evenness of its color differentiates it from ruby, which in addition to its dichroism, often has a splotchy or banded color. Usually, the beauty and rarity of fine red spinel is not reflected in the price asked for it. This may be said also for the intense, orange-red flame spinel, which is the more costly of the two.

The other colors of spinel can only be judged by their individual desirability, since, except for pink or very light violet, the colors tend to be too dark and grayed out. In this regard, it is well to note that the purple and dark-blue stones may be very transparent in transmitted light but grayed out when viewed in a setting. Because of the availability of brighter sapphires and because of the small sizes of spinel in these grayed-out colors, they have never been popular. In all fairness, it should also be noted that an attractive violet to light-purple or bluish green stone will be encountered occasionally. The rare alexandrite like spinel, which changes from a light grayish blue in dayling to an amethystine violet in artificial light, should be judged by the quality of each color and the degree of change under the two light sources. None of these colors commands a high price, unless it has been mistaken for corundum. When the identity is known, the price usually drops.

In general, faceted spinels are much more flawless than the corundum gems, especially when judged with the unaided eye. Visible flaws, therefore, affect value considerably, particularly if they are in the form of cracks or structural defects. Certain stones with especially desirable color may not be affected in value greatly by the presence of inclusions that are visible to the unaided eye; however, this usually does not hold true with less desirable colors.

Because of the rarity of large, rough spinels and the fact that many crystals are twins or flattened octahedral, the finished stones are often cut with other than ideal proportions. The color of a stone may compensate partly for the cutting deficiencies. However, the quality of cutting in stones with other than fine color must be judged by observing the degree of brilliancy and the area of light leakage ("dead" area, or "window") near the culet. The smaller this area, the more desirable the stone from a cutting standpoint. Spinel will take an exceptionally fine polish with ease, lapidaries report; hence, stones with obvious polishing deficiencies are not as valuable as well-polished stones. One factor that does not require consideration when judging spinels is orientation. Since the color is virtually always evenly distributed throughout and there is no dichroism, the lapidary can cut the stone with a view to eliminating flaws and obtaining as large a stone as possible without the complicating factors of orientation and color banding.

Large spinels that is, stones of good color in sizes over ten carats are rare indeed. Museums are happy to have a fine red specimen to display, even if it weighs less than ten carats. In this regard, it is more fortunate than ruby, since very few ten carat rubies are on display. In spite of the rarity of large sizes, the price per carat does not increase as rapidly for the larger sizes as it does for ruby or diamond, although large red spinels may be very desirable and beautiful. Size makes very little difference in price per carat in the colors other than red, orange and pink.