What is the difference between spinel and ruby?

Spinel is at a huge disadvantage as a gemstone, being overshadowed in almost every respect by its luxurious and famous cousins ​​sapphire and ruby, and not being as widely known as it deserves to be. The only varieties that are highly valued as gems closely resemble the real ruby. They are probably often confused, especially since they occur together in nature. Spinels of other colors are not very beautiful and are unlikely to be in great demand. Blue spinel is rare in nature, and its color has a steely tint and is far inferior to the excellent coloring of real sapphires. Spinel is a very hard mineral and is therefore very good for ring inserts. At the same time, it rarely reaches large sizes and sufficient transparency to be used in large jewelry. In mineralogy, the name “spinel” is used for this mineral, and there is no reason not to use it to designate stones used in jewelry; you just need to clarify it, if desired, with definitions indicating the corresponding color. Thus, we can talk about red spinel, orange spinel, etc. The long-established custom of calling red spinel ruby-bale or, less commonly, spinel-ruby cannot be considered successful and should be abandoned. A more acceptable designation for orange-red spinel is the old name “rubicell”. When such names are used, people uninitiated in precious stones may be led to believe that the stones so called are related to red corundum, but the most that can be said about them is that they differ sharply from the latter. Moreover, one should not use confusing terms in the hope of foisting this or that stone on an unsuspecting buyer, calling a purple spinel an oriental amethyst, and a blue spinel a spinel sapphire. Even worse is the opposite practice, when the word “spinel” refers to completely different minerals; for example, Arizona spinel and candy spinel are simply garnets. Such names should never be used by anyone who cares about the accuracy of nomenclature. The purple-red variety should also not be called almandine-spinelyo, since almandine is one of the varieties of garnet. Spinel is a complex oxide of magnesium and aluminum MgAI . Previously, it was classified as an “aluminate,” but it turned out that the crystal structure of this mineral corresponds to the structure of oxides. In this structure, each magnesium ion is surrounded by four oxygen ions, and each aluminum ion is surrounded by six oxygen ions. The structure does not contain complex ions that would justify the old definition of this compound. Spinel itself is one of the final members of the isomorphic series. The magnesium ion can be replaced by a ferrous iron ion (ceylonite, pleonaste, hercynite), a zinc ion (ganospinel, ganite) or a manganese ion. Aluminum can be replaced by chromium (picotite, chromite) or ferric iron (chlorospinel). The word “spinel” comes to us from the French spinelle, but the origin of the latter is unclear. Etymologically, the origin is most likely from the word spinella, a diminutive form of the Latin word spina (thorn), from which the English word spine is derived. However, this is not a very appropriate name for spinel crystals, and it may have originally been applied to a mineral whose crystals have a characteristic needle-shaped shape, and only later was transferred to spinel itself. According to another assumption, the word “spinel” comes from the Greek omvOfjp (spark) and its unusual form oat voc, which was used to designate this mineral in connection with the red color of its most valuable crystals. This assumption is plausible, but not very likely. The word “bale” comes from the Latin word balascus, which in turn comes from the Arabic word “Badakshan” – the name of the area where the most beautiful stones were mined in the Middle Ages. The word rubicell is borrowed from French and appears to be a diminutive form of rubace, another form of rub is. “Pleonaste” is a French word (pleonaste), which comes from the Greek K^eovoccmx; (abundant) – a hint of additional faces that complicate the main octahedron. Picotite is named after Pico de la Peyrouse, who described chrome spinel. Pure spinel is colorless, but such stones are very rare in nature. The beautiful red stones, the so-called bale rubies, are quite well known, and their color is due to the presence of chromium, as is the case with real rubies. The slightly yellowish tint of the rubicella is due to the presence of iron, while manganese may be responsible for the purple-red color. The colors of spinel are so varied that it is difficult to list all the shades between blue and red that this mineral can take. Stones rich in iron are completely opaque, but are sometimes used as decorative. Greenish and brownish varieties are rarely used in jewelry. The various natural colors of spinel are shown in color photo VII. Like diamond, spinel crystallizes in a cubic system and occurs in the form of beautiful octahedra or flat triangular plates (Fig. 123, color photo VII), bifurcated at the corners. These plates are actually twinned octahedra. The way they twin is so characteristic Rice. 123. Spinel crystals: 1 – simple, 2 – twinned. For this mineral, crystals of other minerals twinned according to the same law are called spinel twins. Spinel octahedra are often irregularly developed, so that one face or a pair of opposite faces receive the greatest development. In this case, the spinel crystals are very similar to the crystals of real rubies, since the difference in the angles between the faces is not very large and is indistinguishable to the eye. Due to the fact that spinel belongs to the cubic system, it does not have birefringence and is devoid of dichroism. The pronounced dichroism of ruby ​​and the lack of dichroism of spinel make it easy to distinguish these two gems even when their crystals have a similar appearance. The refractive index of spinel varies widely due to the fact that one of the elements is isomorphically replaced by another without any disruption of the crystal lattice. The value of the refractive index depends on which element is present in the crystal lattice. Since some of these elements have a strong influence on the color of the mineral, variations in the refractive index can be correlated with the color of the stone. Thus, for red stones, the refractive index value varies from 1,715 to 1,735 in accordance with the chromium content. For blue stones, this value ranges from 1,715 to 1,754 according to the zinc content. The refractive indices of stones of other colors range from 1,712 to 1,717; normal value is 1,717. The dispersion is low and for the interval B – G reaches only 0,020. Spinels can fluoresce and give characteristic absorption spectra, although due to variable amounts of isomorphic impurities these phenomena are not constant. The fluorescence of some pink spinels causes the appearance in their spectrum of a group of five bright lines, of which the most intense correspond to wavelengths 6870 and 6750 A and form a clearly distinguishable pair. Purely red spinels give a characteristic absorption spectrum in which the green and yellow parts lying between 4900 and 5950 A are completely absent; in addition, there is a faint band at 6560 A and a more distinct doublet at 6840 and 6855 A, located in the red region, as well as a number of other fuzzy bands. The intensity of the absorption spectrum depends on the density of the color inherent in the stone; for pink stones it is noticeably weaker. Some non-fluorescent red spinels do not produce bands in the absorption spectrum in the red region; The reason for this phenomenon is that the color in this case is not due to chromium, but to other elements. Due to the presence of iron, up to ten bands stand out in the absorption spectrum of blue spinel. However, most of them are indistinct, but the 4590 A band in the blue region is very consistent and distinguishable in stones containing less than 2% iron oxide; it is competed with by another, weaker 4800 A line. The density of transparent spinels ranges from 3,58 to 3,63; in zinc-enriched varieties, the density can reach 4,06. It should be noted that the density of ganite varies from. 4,0 to 4,6; this mineral is practically opaque. In another variety, ceylonite, the density increases with increasing iron content to approximately 4,0; It is a black and opaque variety of spinel. Spinel is softer than sapphire and ruby. Its hardness on the Mohs scale is 8. The shine of spinel is less bright than the shine of these stones. In spinel, imperfect cleavage parallel to the octahedron faces is sometimes discernible, which may be separation caused by twinning. Spinels allow various forms of processing depending on the purpose of the products. Thus, they can be cut with diamond, step or combined cuts, as well as in cabochon form. The only variety of spinel in demand among jewelers is the densely colored red variety, incorrectly called spinel ruby, which closely resembles this mineral, but is considerably inferior in value. The romantic stories of the “Ruby of the Black Prince,” one of the most valuable regalia of the British Crown, and the “Ruby of Timur,” also the property of the British Crown, are given above; both of these stones are actually spinel. The magnificent red stone, now in the Diamond Fund in Moscow, which adorned the crown made for Empress Catherine II in 1762, is also a spinel. It has a deep red color, is clear and transparent and has an irregular shape, as it retains the original oriental cut. It is currently attached with a diamond to a pin, and its weight is estimated at 400-404 carats. The Louvre in Paris houses a beautiful cut spinel weighing 105 metric carats, which in the past adorned the French crown along with other stones. It is said that Tavernier bought three large red spinel stones in India in 1665, weighing about 200 carats each. Two large spinel stones weighing 81 and. 72,5 carats in cut form were shown at the London Exhibition in 1862. In the collection of minerals. British Museum of Natural History There is a large polished octahedron weighing 355 metric carats and a polished spinel pebble weighing 520 metric carats; both of these stones come from Burma. The pebble was captured among other treasures during the capture of the Summer Palace of the Chinese Emperor during the war of 1860-1861. As indicated in the previous chapter, the synthesis of spinel was successfully carried out by the Verneuil method and is of exceptional scientific interest, since as a result of these experiments it was established that alumina in an amount significantly exceeding that required by the theoretical composition of spinel can combine with magnesium oxide, forming a solid solution, without disturbing the crystal structure. Such alumina-enriched spinels do not occur in nature. Refractive index and density increase with increasing alumina content; so, for a colorless material with a ratio of MgO: A1 1:1 density is 3,58, and the refractive index l.715^jipH for the same components 1:4, the corresponding values ​​are 3,62 and 1,728. These synthetic stones can be distinguished from natural spinels by the large values ​​of their physical constants, their anomalous birefringence associated with uneven stress distribution in the crystal structure, and their unnatural shades of color. In general, spinel is most easily recognized by its refractive index, which is noticeably lower than that of ruby, pyrope or almandine, with which it can be confused. However, some pyropes closely resemble spinel, and careful examination of the refractive index and density is necessary to make a confident determination. For example, the refractive index and density values ​​for pyropes from Australia are 1,733 and 3,67, respectively, but for spinel with a similar refractive index (1,734) the density is 3,61. These minerals can also be distinguished by their absorption spectra. Spinel is easily distinguished from the ruby ​​with which it is so often associated and which it is so similar in appearance, due to the lack of birefringence and dichroism that are characteristic of ruby. Noble spinel is found in crystalline limestones and crystalline schists, as well as in alluvial deposits formed due to the erosion of these rocks. Other spinel minerals are components of some igneous rocks, as well as rocks that arose when they were altered. Red spinel and rubies occur together in the Mogok pebbles of Upper Burma. It is surprising that rubies are usually rounded, while spinel is found in the form of regular octahedrons with a very perfect color. In Ceylon, in addition to the red varieties, beautiful purple and blue stones are mined, and, as the name given to the black variety indicates, pleonaste, or ceylonite, is common here. Spinel, along with sapphires and rubies, is present in the alluvial deposits of Chanthabun in Thailand. It was found in the mines of Badakhshan (Pamir). Indeed, the word “bale” is believed to have come from the ancient form of the name of Badakhshan – Balastia. Spinel is an oxide of magnesium and aluminum. The color of the stone is determined by various impurities. For example, the most famous, bright red color is produced by chromium impurities. We are used to red and pink spinels as they are most often used in jewelry, but this stone can actually come in a variety of colors. If the chemical composition contains not only chromium, but also iron, then it acquires an orange color. Iron together with cobalt gives blue color. The varieties of iron itself: bivalent or trivalent, produce black and green stones, respectively.

Color

If earlier jewelers primarily recognized red and pink spinels, today, with the fashion for semi-precious stones (and spinel is included among them), crystals of other colors are increasingly being used. Thus, black spinels are now especially popular, including in works of high jewelry.

Place of Birth

The geography of spinel deposits is also related to its color varieties. The most valuable red stones are mined in Burma and the Pamir Mountains (Afghanistan, Tajikistan). Pink stones are brought from Afghanistan, as well as from Tanzania. Blue spinels come from Sri Lanka and Vietnam, including the so-called cobalt spinels, which are very similar to sapphires. Sri Lanka produces green and black stones. We also have spinels of different colors: in Yakutia, the Baikal region and the Urals. Spinel deposits are often found adjacent to ruby ​​and sapphire deposits, which has increased the confusion in the past.

Noble Spinels

Gem-quality spinels are called noble. The rarest and most valuable variety is considered to be the bale-ruby. These stones are ruby ​​red or deep pink in color. The name of the stone was given according to the Europeanized version of the name of the province of Badakhshan in Afghanistan, where these spinels were mined since ancient times. The best examples (in addition to color, the purity and size of the stone are taken into account) can sell for $20-30 thousand per carat, thereby approaching the prices of the “big four” gemstones: diamonds, sapphires, rubies and emeralds. Spinel is considered a separate mineral, so along with bale ruby, its other varieties were also given independent names. Thus, rubicella are yellow spinels, ceylonites are black and green, and suricus are purple and blue-gray.

Inclusions

Although the transparency and purity of the stone increase its price, the presence of small inclusions, visible through a standard magnifying glass at 10x magnification, will confirm that the stone is natural and not synthetic. Spinels have been grown in laboratories for a long time, and laboratory samples, unlike natural ones, are almost flawless. However, synthetic stones are valued significantly less than natural ones, because deposits of noble spinel are quickly selected, and over time this stone will become a rarity. Spinel is counterfeited using glass or cheaper (compared to spinel) garnets. But much more often it is used as a counterfeit, replacing rubies or sapphires.

History

They were able to distinguish these stones in the East. Thus, one of the first mentions of spinel dates back to the 10th century and to the treatises of the Persian scientist Al-Biruni, who names the main characteristics of the stone (color, shine) and notes that in comparison with ruby ​​it is not so hard. (Although on the 7,5-point Mohs scale the hardness of spinel is estimated at 8-XNUMX). Europeans did not distinguish between rubies and spinels until the 13th century. In the 400th century, Marco Polo mentioned spinels in his “travelogue,” but he did not draw a clear line between them and rubies. In Rus’, all red precious stones were usually called lalas. And the central XNUMX-carat stone in the Great Imperial Crown, with which, starting with Catherine II, all Russian monarchs were crowned kings, was identified as a spinel only after the revolution, when Academician Fersman cataloged the royal jewelry.

Effects

In nature, this mineral grows in the form of sharp crystals. Hence its name: in Latin, “back” is a thorn. Although there is another, less popular version of the origin of the name of the stone, which refers it to the Greek language, where “spinos” is a spark. The latest version can also be associated with the interesting, almost magical properties of spinel. They appear rarely, and such samples immediately add value. Some stones have a so-called alexandrite effect: they change color depending on the light. For example, in natural light the stone is blue, but under a lamp it turns purple. Also, the color may change from purple to red. Spinels have luminescence and glow in the dark. And stones with asterism seem to contain a star. They try to cut such samples in a special way to emphasize these almost transformations. In ancient times, spinels were considered the stones of warriors. Thus, the most famous in the world, the 5-centimeter spinel, which now adorns the crown of English monarchs, once crowned their helmets. She went down in history under the name “Ruby of the Black Prince.” King Edward was called the Black Prince, and he got the stone from the Castilians. Edward was the first to wear it as a talisman, attaching it to his helmet. Subsequently, the “Ruby” actually saved the life of another monarch: the enemy’s sword broke on the helmet of Henry V, and he remained unharmed. The East has its own symbolism: there spinel is considered a stone of love, shown to both women and men. A common European tradition calls spinel a talisman for travelers. Lithotherapists suggest using it in the treatment of eye and – what is especially important now – infectious diseases.

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