Corundum (group of corundums) - colorless corundums, rubies and sapphires

Corundum (group of corundums)
Chemical formula: Al2O3.
Chemical composition (%): Al - 53.2; О - 46,8.
Shingonia: triclinic, the habit of crystals in the form of hexagonal flat plates (ruby); Also columnar, pyramidal (sapphire and other varieties of corundum). As a rock-forming mineral is also found in solid opaque granular masses.
Color: various, due to impurities, isomorphously replacing aluminum (chromium, iron, titanium).
Hardness: 9 (the hardest after diamond).
Density: 3.96-4.05 g / cm3 (ruby), 3.99-4 g / cm3 (sapphire).
Cleavage: absent.
Glitter: glass.
Transparency: the jewelry corundum of all colors is transparent. Light refraction: n = 1.766-1.774.
Birefringence: 0.008.
Dispersion: 0.018.
Pleochroism: yellow-carmine-red (ruby), blue (sapphire).
Dichroism: distinct, ruby ​​red for a regular ray and pale yellowish-red for an unusual; Sapphire, respectively, deep and yellowish-blue; In yellow corundum - the difference in color in its depth.
Adsorption (clear lines): 694.2, 692.8, 476.5, 475.0, 468.3 nm - for ruby; 471, 460, 450 nm for the sapphire.
Luminescence: carmine-red for ruby, bluish-violet for sapphire; Yellow, orange for yellow corundum. Chromium in ruby ​​is chromium and phosphor. Rays of light excite ions of trivalent chromium, which begin to luminesce, forming a luminous red color.

Red and blue corundums (rubies and sapphires). The Polar Urals.

The admixture of chromium imparts a purple-red color to the ruby, while the brightness of the color depends on the amount of chromium. Thus, in the best blood-red crystals from Burma chromium contains about 2, sometimes up to 4%, and in pale pink rubies only tenths of a percent. The impurities of titanium chromophores are blue color (sapphire), iron - yellow (padparadzha), and iron and manganese-brown and pink. Sometimes zoned crystals of corundum are found in which alternating bands of purple, violet, yellow and blue alternate, etc. Absolutely colorless crystals are not found.

The name of the mineral comes from the Sanskrit Kuruwinda- ruby. Red ruby ​​and blue sapphire are the most famous gemstones of the corundum group. Sapphires are not only blue stones, but also yellow, green, purple, pink and colorless.

By corundum in engineering and trade, we mean relatively pure varieties of it, used as an abrasive, as well as with an admixture of magnetite and other minerals (emery).

Theophrastus (372-287 BC) called the red corundum ruby. Some researchers called the ruby ​​the Latin "rubens" - red, blushing. In the XI century. Al-Biruni wrote about the ruby, as the first among the precious stones, the most beautiful and expensive. He referred to the highest grade garnet and saffron, then purple, purple, purple - that in his presentation corresponded to the color of embers and embers, the flame of a candle, meat, pomegranate. Pink rubies in the Middle Ages were called carbuncles.

Sapphire is an expensive and beautiful blue gem. In ancient times it was considered a stone of contemplation, a symbol of the heavenly dome (from the Latin sapphires - blue). Sapphires before the XIX century. Called all the blue gems, and then only blue corundums. In Russia ruby ​​and sapphire - "yakhont chervleny" and "yakhont azure" - were considered as varieties of one mineral.

The color of bright cornflower blue crystals from Burma and Thailand is called "sonorous", and blue crystals with a silky shine are royal. Sapphires of Australia - black, inky color with a greenish tinge. (Similar sapphires have recently been discovered in Tanzania). Sapphires of the Urals usually have a gray tinge.

Sapphire is characterized by asterism (from the Greek star): on the polished stone clearly visible light starlet with three, six or twelve rays. The difference with the blurry image of the rays is called zhirozol. This optical effect is associated with the thin needles of rutile, oriented in three directions at an angle of 120 ^{° to} each other. Rubies with a similar effect are much less common. Corundums with the manifestation of asterism are usually processed in the form of a cabochon and are in high demand. The formation of corundums is associated with various processes: magmatic, contact-metasomatic (skarn), metamorphogenic.

Industrial concentrations of jewelry varieties are also confined to exogenous formations; They arise as a result of the destruction of indigenous sources. The manifestations of ruby ​​in the bedrock among the deepmetamorphosed rock complex in biotitflogopitic micas, which are accompanied by plagioclase, talc, chlorite, serpentinite (Ural), have been revealed in the CIS and Russia. Dark-red crystals of ruby ​​are usually noticed here, usually in the size of 0.5-1 cm, sometimes up to 8 cm in diameter. The shape of the crystals is thick hexagonal plaques with a flat side of the pinacoid. The finest crystals of ruby ​​end with a brilliant edge. These crystals are very fissured and therefore can still be used as a collection material.

Corundum (crimson, 1 cm), Azov, south-east of Ukraine, the CIS.
, Ukraine, minerals (August 29, 2011, 08:49).

Mineralogical finds of ruby ​​are associated with ancient regional metamorphosed rocks rich in alumina (ruby-containing marbles) and corundum plagioclasites in the South-Western Pamirs, sometimes in association with spinel, and sapphires with the ancient carbonate strata of the Baikal region and alluvial deposits of the Urals.

Ruby is also found in Kazakhstan. Large rubies are rare and highly valued, large sapphires are more common. Yellow and other varieties of noble corundum are in low demand. Rubies and sapphires are processed in the form of diamond cut and are used in expensive jewelry along with diamonds, emeralds, pearls.

The Diamond Fund of Russia keeps a unique sapphire of thick cornflower blue, weighing 258.18 carats, a diameter of 3.9-3.4 cm, and a height of 2.2 cm. The surface of the stone contains more than a hundred hexagonal faces. In the regalia of the power of the Russian kings - the scepter and the power - more than 219 rubies, 326 diamonds, 48 ​​emeralds; On the famous "Cap of Monomakh" - four crystals of large rubies, noble spinel and emeralds.

Popular gems, which are represented on the territory of the CIS (former USSR)