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Russian Gemstones Encyclopedia

Vladimir Bukanov. Russian Gemstones Encyclopedia

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

SPINEL, GROUP: SPINEL, GAHNITE, HERCYNITE, MAGNETITE, CHROMITE


SPINEL group (Spinell—Spinelle—Ўпинель) Theophrastus, 315 B.C., from Gk. “spinther” – sparkle, alludes to its luster crystals.

Composition & Properties. Among the varieties of spinel there is spinel or alumospinel and also zinc spinel – gahnite, ferrous spinel – hercynite, ferrospinel – magnetite; chromspinel – chromite. Oxide – MgAl2O4, cubic system. Admixtures → color: chromium (Cr3+) – red, rose; chromium plus iron (Cr3++Fe3+) – orange-yellow, iron (Fe2+) – light blue, violet; iron plus cobalt (Fe2++Co2+) – dark blue; iron (Fe3+) – green. Hardness 7.5-8. Density 3.6-4.1. Cleavage indistinct. Glass luster. Fragile. No joints, but sometimes separateness is obvious.

Gahnite – ZnAl2O4 (von Moll, 1807), after Johann Gottlieb Gahn (1745-1818), swedish chemist and mineralogist, who discovered the mineral. Density 4.0-4.6. Gahnite makes a row of constant mixing with alumospinel. It is found in crystals, not rarely, twin crystals, it can also form grained aggregates. Its color depends on iron admixture – yellow, green, blue to black, brown. Intermediate variety (in content) of gahnospinel, or automolit is represented with transparent crystals of light blue, blue-violet and violet color, sometimes with asterism in star gahnite.

Hercynite – FeAl2O4 (F.X.M. Zippe, 1847), for Silva Hercynia, the Lat. name for the Bohemian Forest where the mineral was discovered. Netschetin, Bohemia (historical region), Czech Rep.. Density 3.9-4.4, of black color produces the row of constant mixing with alumospinel, gahnite and chromite. Transit form from hercynite to spinel is called pleonaste, and to chromite-picotite. This black spinel forms translucent to transparent, dark green to black and dark brown, yellowish and greenish-brownish crystals, which are used sometimes for cut.

Magnetite – Fe2+Fe23+O4 was known to the ancient Greeks, who called this mineral magnetis (magnet), because it was found in two places in Lydia, Magnesia and Herakleia. Density 5.5-6. Density 4.8-5.3. Metal luster. Fragile, it is black, non-transparent with metal luster. It possesses strong magnet properties. Chromite – Fe2+Cr2O4 (Haidinger, 1845), the name reflects its chromium content. The most often found are magnesiochromite – MgCr2O4; chromepicotite – (Mg,Fe)(Cr,Al)2O4 and alumochromite – Fe(Cr,Al)2O4 Hardness 5.5-7.5. Density 4.2-5.1. No joints. Fragile, it is black, non-transparent with metal luster. Opposite to magnetite it has no magnetism.

Spinel is found in well-formed isometric crystals and irregular grains. Their size is usually not very big, but sometimes they reach 30 cm. in cross-section and 100 kg. Transparent to nontransparent. Color depends on admixtures; in a pure form it is colorless. According its color they classify such varieties of noble spinel: fire-red – ruby spinel, dark rose – balas ruby, pale rose – iris spinel, orange-red – rubicelle, dark green – chlorospinel, blue and light blue – sapphire spinel, violet-red – almandine spinel or almandine ruby; violet – oriental amethyst and very rarely – colorless spinel. Its color is not stable, it weakens in light. Sometimes star spinel with four or six-radial star is found, its asterism is connected with regular oriented inclusions of rutile, sillimanite and sometimes titanite. These inclusions produce the effect of silk in noble spinel like in sapphire or ruby, and also the effect moonstone. In the collection of the Smiths. Inst., Wash., there is spinel with pseudoasterism, in which it is seen seven-radial star under the light. The alexandrite effect with violet color was also observed in spinel in daylight and with red-violet color – in artificial light.

Deposits. Spinel as high-temperature mineral is formed in magnesian skarns, contact-metasomatic deposits. It is extracted usually from placers. In Russia deposits of noble spinel are not found yet. In the South Urals ferrous spinel was found in iron-ore skarns of the Blagodat’ Mt., near Magnitogorsk; chlorospinel – in the Shishimskie Mts., near Zlatoust. Separate big crystals of dark green and black spinel reached here 16 kg. At the Nikolaevo-Maximiluanovskaya mine a crystal of hercynite 21 kg. was found. It is exhibited in the Museum of the Kazakh University now. There was also found crystals of pleonaste of brownish-black color, 10 kg. and 16 cm. along the edge. The more perspective region for spinel searching is supposed to be East Siberia. In calciphyres and marbles of Pribaikalia, from the region of Slyudyanka in the south to the Ol’khon Is. and the St Nose Penin. in the north, there were separate findings of rose and dark blue spinel, up to 4 cm. in cross-section. In Yakutia, in calciphyres of the Aldan Plateu, at the deposits: Emel’dzhak and Katalakh, crystals of black spinel – hercynite reached 30 cm. in cross-section. In Tadzhikistan, in the West Pamirs Mts., 37 km. to the south from Khorugh (Khorog), there is the large Kukh-I-lal deposit of noble spinel. Its exploitation was opened in the 7th cent. A.D., and after a long pause it was reopened in 1972. Red spinel in crystals, up to 20 cm. in cross-section, often together with transparent clinohumite, is found in loose material of cavities in zones of metasomatic alteration and rock shattering in forsterite marbles. Crystals of spinel usually are crackled, so material of gem-quality is collected from debris, which are sometimes up to 500-750 ct. In 1985, a unique crystal of gem-quality was found; it was thick rose in color, 15x17x20 cm. in size and 27,900 ct. (5.58 kg.). It was valued at $300,000. In 1989, 30 km. to the southeast from Kukh-I-lal, on the Garon-Dara River one more deposit of violet noble spinel with alexandrite effect was discovered. Its transparent crystals were no more than 1.5 cm.

In Sweden, at the Åker deposit, Södermanland, the first gahnite was found – pale blue and pearl-gray in color. Findings of spinel are known also in Finland, on the Parainen (form. Pargas) Is., in Italy – in South Tyrol, at one deposit in the Fassa Valley. In Kenya deposits of blue noble spinel are situated near Ngorigay she Hills. In Tanzania rose, red and orange spinel at the deposits Umba, Magagoni, Makhenge and Tunduru, were extracted. In the southern part of Madagascar polychromatic noble spinel in crystals up to 15 cm. at the deposit Ambatomainti, near Betroka, was extracted; also at Beraketa in black crystals up to 24 cm. are known. In Western Australia gahnite is extracted at placers at the Nevoria deposit, and in New Zealand, there is also pleonaste together with sapphire. Among deposits of South-East Asia the best red noble spinel occurs in marbles in the Mogok region, Myanmar (Burma). In this country hercynite together with ruby at the deposit Mong Khsy was extracted. In Thailand extraction of red and blue noble spinel is held at placers of the southern part of the country together with ruby and sapphire near Chantkhabury and Trata. Besides, such exploitation works are held in the eastern part of the country, in the Sisaket Prov.; in the northern part – in the Muang Phre Prov.; in the western part, near the border with Myanmar, – in the Kanchanabury Prov. Next in importance are placers of Sri Lanka in the Ratnapura area, where different in color spinels are extracts, including star spinel, spinel with alexandrite effect and sapphire-blue cobalt spinel in crystal up to 2 cm. In Vietnam, at Lüc Yên, Yên Bai Prov., there are deposits of red, rose and pale blue spinel; as well, too in China, and Cambodia. In India red, blue and violet crystals up to 5 cm.; in the Jammu and Kashmir State (disputed area), in marble at Alïãbãd, Hunza Valley, were extracted. In Afghanistan red, blue and violet spinel at the Jegdalek deposit occurs. In Brazil spinel is extracted as additional material at the diamond fields of the states of Bahia, Minas Gerais and San Paulo. It is mainly red, blue and colorless spinel. In Mexico hercynite is extracted at placers. In the U.S.A. bluish-green ceylonite is found at one deposit in New Jersey, and in the New York State crystal of pleonaste up to 25 cm. (26.8 kg.) at Amity, Orange Co., was found. In Maine crystals gahnite up to 7 cm. occurs in pegmatite at Mt. Apatite, Andrascoggin Co. In Canada black spinel was extracted at the deposit South Burgess, Ontario Prov., and violet spinel in crystals no more than 1 cm. at the deposit Cameron quarry. In the North-West Territories a deposit of polychromatic spinel was discovered on the McDonald Is. In the Québec Prov. crystals light blue spinel up to 6 cm. at the deposit Parker mine, near Ottawa (Bytown form.), was found.

Ornamental rocks. In Russia, in the West Sayan Mts., in the place of contact of quartzites and serpentinites, in the zone of mélange of the Ijimskiy hyperbasite massif, spinel-uvarovite rock is discovered. It is an ornamental stone of bright green color, easy to polish. In Uzbekistan such rock is corundum-spinel gabbro – alaite. At light green background of its pauro-grained diopside and plagioclase there are bright spots of red spinel, which take up to 30% of rock material. In grained aggregate of spinel there are separate inclusions of ruby, up to 2 cm.2 in size. Birbirites as an ornamental stone is found in Ethiopia, Kazakhstan and Australia – in the Rockhamton Dist., Queensland State; as well as in Brazil – at the Liberdade mine, Minas Gerais.

Gahnite was described in Russia at the Urals, in talcum schists Shishimskaya mine of the Shishimskie Mts. in 1833. Later it was found in pegmatites of the Ilmeny Mts., in crystals not more than 1 cm. In North Pribaikalia gahnite was extracted as additional material at the Kholodnenskoye deposit. In Europe its findings are known in Sweden, in the Falun region, Dalarnas; in Germany – at the deposits Silberberg and Bodenmais, Bavaria. In the southern part of Madagascar, in pegmatites near Betroka a crystal of gahnite 100 kg. was found. In Australia it was extracted as additional material at the deposit Broken Hill in New South Wales. In the U.S.A. gahnite is not rare in pegmaties of California, Colorado, New Jersey and Maryland.

Magnetite is found at deposits of different genetic types as well formed crystals, as mass inclusions in gemstones and ornamental stones, which have decorative significance, and as well as rock-forming mineral of some ornamental stones. In Russia in the Middle Urals near Kachkanar it was found in the crystal up to 10 cm.; and also of the Shabry deposit shape of big crystals in chlorite-schists; in the Murmansk Region – in carbonatites of the Kovdor deposit up to 5 cm. In East Greenland it is reached up to 15 cm. in Gardiners complex, near Kangerdluarssuk Fjord, Denmark. In Sweden, at the deposit Västanfors, Västmanland, a crystal of magnetite, 25 cm. in size, was found. In Romania in garnet skarn of the deposit Ocna de Fier crystals of magnetite reach 6 cm. in size. In Italy a crystal of magnetite, 15 cm. in size at the deposit Traversella, Piemonte, was found. In Mozambique magnetite crystals up to 27 cm. in size in the Tete Dist. they found. As small inclusions magnetite is typical for nephrite and serpentinite. In these ornamental stones it produces decorative spots. As rock-forming mineral of ornamental stones, magnetite is represented in ferrous quartzites, or jaspillite, as well as in magnetic iron ores. Among them, there is fine-bedded quartz-magnetite-hematite rock from the Griquatown deposit, South Africa; in cabochon oriented along its layers it has chatoyancy, that’s why it is called iron-eye. Magnetite jade, or black jade is of the same type; it consists of magnetite grains covered with a film of native gold. In the U.S.A. this jade in California was found, it has been used in jewelry since 1970. In Brazil, on deposit Jaguaraçú, Minas Gerais, found crystals magnetite up to 10 cm. Muschketowite (mouchkétovite) – pseudomorph of magnetite on hematite can have its significance for collectors.

Chromite was found at first in Russia, in the Urals in 1797, where it received the name chrome iron. It forms banded leopard ore. Pure chrome without admixtures is found only in meteorites. In the western part of Africa, in Sierra Leone, at the deposit Hangha they found crystals of chromite about 3 cm. in size. Usually it is found as inclusions in magmatic rocks of ultrabasic content, in products of their transformation under metasomatism – birbirites; in their weathering crust – serpentinites, and also in metasomatics in contacts with them – nephrites. Birbirite is bright colored quartz rock; according its content it is chromite-containing quartzite. It is easy to polish; relict grains of chromite give it decorative significance.

Synonyms. Spinel – Spinel moonstone | Talc spinel | Spinelite | Spinello | Sparkling stone.

According its color: red and rose spinel: – Alabandine (alabandite), after the Alabanda – ancient center of gemstone trade in Turkey | Anthrax, obs. | Ballagius, from Ballaxian (Balaghat), ancient country, now Badakhshan in Tadzhikistan; Germ. ballas, Fr. balais, Engl. balas | Carbuncle, obs. | Red diamond | Lhal, Arabic, in the East all precious red stones; in Russia – Chinese spinel and not very expensive red stones | Lychnis, after “lychnos”, Lat. – lamp. | Purple mallow | Aluminum iron ore | Purpurspinell, Germ. | Rezircon, trade name | Ballas ruby | ~ ruby: almandine ~, garnet ~, Siam ~, mistake; spinel ~. |~ spinel: Bala ~, fire ~, flame ~, purple ~, red ~.

Orange and yellow spinel – Essigspinel, Germ. | Yellow ruby | Spinaria, Old Russ. | ~ spinel: Chinese ~, hyacinth ~, treacly ~, vanadium ~, vinegary ~. | Vermeil, trade name of orange-red garnet, spinel or zircon.

Gahnite – Akerite, after the discovery location in marbles of Aker, Södermanland, Sweden | Automolit, from Gk. “automolos” – transfer | Octahedral corundum | Fahlunite, after the discovery location near Fahlun, Sweden.

Green spinel – Siam aquamarine | Dirigem | Oriental emerald | Chrysolite spinel | Green spinel | Zinc spinel.

Blue spinel – Hope sapphire | Blue spinel | Cobalt spinel | Lazur spinel | Zarafina.

Violet spinel – Purple spinel | Surovik, Old Russ.

Hercynite – Candit, after the discovery location by Candy, Sri Lanka | Ceylonite, after the discovery location on Sri Lanka (form. Ceylon) | Chrysomelane, after its color, from Greek “chrysos” – gold and “melanos” – black | Ferropicotite | Picotite, after the name of Fr. naturalist Pico de la Peiru, who described it | Pleonaste, from Gk. “pleonasmos” – plenty, because of its multiple faced forms | Skorian |~ spinel: black ~, ferrous ~, chrome-iron ~, magnesian-iron ~.

Magnetite – Aimantine (aimant), from Fr. “aimer” – to love | Chumbaka, in Sanskrit – the kisser | Black diamond | Heraclean | Octahedral iron luster | Lodestone (loadstone) magnetite possessing polarity, from “lode” (Middle Engl. “lod”, “lode”, from Old Engl. “lãd” – way, course, journey) | Magnatis (magnet), obs. | Muschketowite (mouchkétovite), after of Russ. geologist I.V. Mushketov (1850—1902) | Octahedral iron ore | T’ su shi, in Chinese loving stone | Sideritis | Iron spinel |~ stone: Heraclean ~, Hercules ~, magnetic ~, siliceous ~. | Titanomagnetite.

Chromite – Alumochromite, blue ferrochrompicotite, hercynite-chromite | Chrome-picotite – blue, syn. alumoberezovite, chrompleonaste, chrome-ceylonite, picrochromite | Birbirite, after the discovery location on the Byrbyr River in Ethiopia | Chromic iron ore | Chrome-spinel – blue chromspinel | Magnesiochromite, syn. blue berezovite.

Quality improvement. Dark color of iron-containing spinel can be weakened through its oxidation in the process of annealing. Under the temperature of 1000°C thick blue color comes into light blue-green one, and less than 1400°C – into olive-green and then into yellow one.

Cut Gems. As a precious stone, noble spinel has been known since the most ancient times, when its red varieties were not differed from ruby. The examples of it are two unique stones from the treasuring house of Great Britain, which have been being treated as rubies for long. The first of them is mounted in the imperial crown; it’s the “Black Prince Ruby”. It is an oval polished stone of irregular form, 170 ct. and 5x6 cm. in size. The second one is the “Ruby of Tamerlan”, a stone of ancient Indian faceting 352.5 ct. with engraved names of its owners. This one, together with three smaller spinels, 96, 74 and 35 ct., is mounted in the necklace, which is kept in the Indian chamber of the Buckingham Palace in London. The same mistake was made in Russia with the unique spinel 398.72 ct., which was mounted in the upper part of the imperial crown of Catherine the Great. This stone was bought as a ruby in Beijing in 1676. It is one of the seven historical gemstones of the D.F.R., Moscow; it is on the “Cap of Monomach” – the first tsar crown in Russia. Another jewelry from the D.F.R., Moscow, it is a pair of brilliant brooches, 4.5 cm. wide, with red spinels – six stones are mounted in each one. Another unique work of art is a ball set with a bow-sclavage, 11.5x11 cm. in size, and pendants, and 6.7x5 cm. in size. There are 39 big thick rose spinels mounted with brilliants in this set. In the D.F.R., Moscow, there are also two big spinels mounted in gold: one is 100 ct., another one is 56 ct. In the Gold chamber of the State Hermitage, SPb., there is a pendant with spinel, like a big plum in size, which belonged to a British corsair Francis Drake.

In Czech, in the most ancient crown of St Wenceslas from the Museum of Prague Castle, beside rubies and sapphires, there were 45 red spinels, the biggest of which is 100 ct. In France, in the royal treasury there were three red spinels: 124.3; 211.6 and 247.6 ct. One of them (211.6 ct.), mounted in a brooch, had been faceted anew in 1749, after it became 107.9 car and has been inset into the order of “The Golden Fleece”. Now it is exhibited in the Louvre, Paris. Another stone (247.6 ct.) had been cut as a dove for the order of “The Spiritus Sanctus” in 1757, and in 1792 it has been stolen and lost from sight. Perhaps, these three spinels were taken to Europe from India by a jeweler J.-B. Tavernier in 1665. Besides, in the Louvre, Paris, there is a red spinel “Brittany Coast”, 105 ct., made as a cut adornment in the shape of a dragon. It belonged to the Queen Anne of Brittany. Among the unique gemstones of the British Museum, London, there are two Burmese spinels: one of them in the shape of a polished pebble, 520 ct.; another one – a polished octahedron, 355 ct. In Teheran, in the treasury of the Iranian Shahs, three unique spinels were kept: one of them – more than 500 ct., the second one – about 225 ct., the third one – more than 200 ct. Among the unique facet stones of the AMNH, N.Y., there is a red spinel from Sri Lanka, 71.5 ct. In the Smiths. Inst., Wash., there are two spinels from Sri Lanka: a pale purple one – 45.8 ct. and a rose-violet one – 29.7 ct.; and also two spinels from Myanmar: an indigo-blue one – 36.1 ct. and a red one – 34 ct.

Spinel is optical isotropic, that’s why it doesn’t demand special orientation of a stone in facet. It can be as brilliant-cut, stepped-cut and combined-cut of faceting, and also it is processed in cabochons. Usually stones good for cut are not more than 15 ct., and stones of lower quality are processed in cabochons and reach 40 ct. and even more. Non-transparent black varieties, such as hercynite, magnetite and chromite are rarely used in jewelry, mainly for ritual (funeral) purposes. It is known, that Isaac Newton wore a signet with magnetite.

Legends. Noble spinel is supposed to be a love talisman. May be, that’s why it is often cut in the shape of a heart for the rings of the collection of the State Hermitage, SPb. As an amulet, noble spinel protects against nightmares and soul diseases. It is good for the Venus signs: Taurus, Libra and Pisces. Magnetite was known as a stone of sorcerers in the ancient times, and in the Middle Ages – as a stone of alchemists. It was used for healing of heart wounds and for the love potion. Greek believed it was a talisman of Hermes – a god of trade and a protector of thieves and swindlers. In China it is a love stone, which attracts attention and good feelings to its owner. Magnet properties of this mineral are used in modern medicine.

Synthesis. Synthetic spinel was made at first in France, in 1848. The classical method of its manufacturing is non-crucible growing of crystals in the shape of “bull” in the flame of a burner according Verneille. The content of synthetic spinel includes higher percentage of Al2O3 to avoid fragility. So, since 1925, spinel has been being manufactured, mainly for the imitation of precious gemstones. Another method of synthesis if a crucible one according Czochralski – pulling of a crystal out of melted material. Recently, in 1990s, in Novosibirsk manufacturing of synthetic spinel was organized with the method of flux from solution in melted material of lead fluoride under the temperature of 1200°C. With this method in 6 or 7 days they receive crystals up to 2.5 cm. in cross-section. Two latest mentioned methods give a possibility to manufacture spinel, which is equal to its natural formula. Different color is made with combinations of admixtures – chromophores. They received synthetic spinel with alexandrite effect – alexandrine or alexandrine synthetic spinel, synthetic gahnite and Inamori padparadschah synthetic spinel. Colorless leucospinel after annealing receives the effect of moonstone. The biggest similarity with genuine spinel has such stones as: red and especially blue spinel, looking like cobalt spinel from Sri Lanka. Spinels of other colors have inner tension, which is obvious in their uneven color when turning a stone around – the effect of a stripped cat. All of them, opposite to genuine spinel have well represented joints. Visually synthetic spinel can be differed from natural one because of the presence of flux inclusions and small parts of metal. Manufacturers choose names of synthetic spinels after their color and according the ideas of people. Besides, there are several more or less stable names; some of them are given to distinguish color varieties. Beside spinel crystals, they manufacture synthetic polycrystalline material – degussit, mainly blue for the imitation of lazurite. Among other artificial jewelry materials with spinel structure synthetic magnesium gallate is produced – (MgGa2O4), it is used as a substitute for synthetic garnets.

Synonyms. Colorless synthetic spinel – Alumag | Jourado diamond | Corundolite | Lestergem | Magalux | Radient | Rozircon | White spinel | Hope stone | Strongite | Thermospinell | Zircolite.

Red (chromium admixture) – Almandite | Berylit..

Orange (chromium and manganese admixture) – Inamori padparadschah synthetic spinel.

Yellow (iron and manganese admixture) – Berigem (perigem), or erinoid.

Yellow-green – Emerada (emerala, esmeralda).

Green (manganese, cobalt, chromium and titanium admixture) – Emeraldine | Erinide (erinite) | Derigem | Green tourmaline.

Green-blue and light blue (cobalt and chromium admixture) – Aquagem | Azurlite | Elbaite | Zirconium spinel | Blue zircon.

Blue (cobalt and nickel admixture) – Hope sapphire | Sinterspinell, Germ. or Degussit, from Germ. firm Degussa (Deutsche Gold- und Silberscheideanstalt, Frankfurt) | Cobalt spinel | Nicel spinel | Zirconium spinel.

Violet (iron and cobalt admixture) – Ultralite.

Similar Gemstones & Imitations. Similar in color with spinel there are such gemstones as aquamarine, alexandrite, amethyst, hyacinth, garnet, ruby, sapphire, topaz, tourmaline. Opposite to ruby spinel has no dichroism. From garnet it differs with more high hardness. Other similar gemstones differ from it with optical properties. As imitations of spinel, such stones are used: garnet, synthetic spinel, synthetic strontium titanate, doublet with synthetic strontium titanatediarita and also glass. In mosaic triplet upper and lower parts are represented with synthetic spinel, and fine three-colored film is glued between them. The same principle is used for imitation of star spinel.

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