Rubin i szafir – Korund.
Transparent types of corundum are precious gemstones, have been used for ornamental purposes for a very long time. The red variety is called ruby, blue or other - sapphire. Fine crystalline, compact, granular varieties of corundum are used for industrial purposes as an abrasive and polishing material.
The name corundum comes from the Sanskrit kuruwinda, ruby from Latin rubens - red, and sapphire - from the Arabic sapphire.
Chemical properties. Corundum is aluminum oxide Al2O3, sometimes taking away small admixtures of chromium, titanium or iron. Transparent crystalline varieties are exceptionally pure in terms of chemical composition. The color of corundum depends on chemical admixtures. Very small admixtures of chromium color the corundum red, titanium - blue, iron - yellow and brown to black. Considerable amounts of admixtures contain compact, fine-crystalline varieties. They are used as an abrasive (emery). Corundum does not melt in the blower flame; it does not dissolve in acids.
Character. Corundum crystals, often of considerable size, belong to a trigonal system. The walls of the diatribal column are the most common, rhombohedra, pyramids and base dihedral. Walls of the column, of the pyramids and base dihedral are often distinctly grooved. Larger crystals are sometimes rounded and barrel-shaped. Twin adhesions are not uncommon; the most common are twins, whose twin plane is the face of the rhombohedron; sometimes we find penetrating twins, Multiple twins are also quite common. Sometimes a twin is only visible under a microscope. Ruby and sapphire usually appear as distinct crystals, other types of corundum, on the other hand, usually form dense and granular masses.
Physical properties. Corundum has no cleavage, however, some separation can be observed parallel to the base double-wall, a nieraz, due to multiple twins, parallel to the walls of the rhombohedron. Corundum has a shell fracture, hardness 9 on the Mohs scale. It is the hardest mineral after diamond, very resistant to mechanical factors. Depending on where the corundum is found, some differences in its hardness can sometimes be found.
The density of the corundum is 3.9-4.1 g / cm³, its gloss is glassy; Polished ruby and sapphires have a shine similar to diamond. Noble varieties are transparent, the common corundum is opaque.
The color of corundum varies. Common corundum is mostly gray, transparent varieties have different colors. There are red ones next to the colorless ones, blue, yellow or purple. Some varieties exposed to ultraviolet rays show luminescence. When heated, some sapphires may lose their color or take on a lighter shade.
Corundum is a mineral that refracts light twice. Its refractive indices are: nω = 1,763 — 1,772, nε = 1,579 — 1,768, double light refraction is slight (0,008), optical nature negative. The light dispersion in the cord is poor (0,018), therefore rubies and sapphires have relatively little fire compared to diamond and some other gemstones.
Korund, especially with darker colors, is characterized by a strong dichroism. There are dark red and pale red colors in the ruby, in sapphire - blue and yellowish blue. The phenomenon of dichroism most clearly occurs in directions perpendicular to the main axis of the crystals; its degree depends on the color of the stone.
Some corundum, like alexandrite, show a certain color difference in daylight and artificial light. In other varieties, the phenomenon of asterism occurs, caused by the presence of small infixes (inclusions) or the internal structure of the stones. Stated, that asterism in rubies is caused by tiny acicular rutile crystals, in sapphires - empty channels. They are positioned correctly and intersect at an angle of 120 °. Rubies and sapphires, in which such "stars" occur are called starry.
The proper color of rubies is dark red, sometimes purple-red. The most valued are red varieties with a slight bluish tinge; this color is sometimes referred to as the red of pigeon blood (ang. pigeons-blood red, not M. Pigeon blood). This is the color of the rubies from Burma, while Ceylon rubies are usually slightly lighter and have a brownish tinge due to the admixture of chromium and iron.
Colorless corundums are called white sapphires, that is, leucosafirs, and colored blue or otherwise are referred to as sapphires. They may be yellow, golden or green; there are even pink sapphires. Red-yellow varieties have long been called padparajas. The most valued color of sapphires is a shade of cornflower, often called cashmere blue (ang. Kashmir blue). This is the color of the sapphires from Kashmir, Burma and Siam. Ceylon sapphires are lighter in color, and Australian sapphires much darker, sometimes with a greenish shade.
The color distribution in noble types of corundum is not always even. There are streaks and stripes of different shades. The alternating dark blue and colorless stripes occur especially in Australian sapphires. In such cases, grinders place the most heavily colored part of the stone on the top part of the cut, and colorless on the underside. Stones so differently colored can sometimes be mistaken for doublets, i.e. artificial stones consisting of 2 parts. Partially blue colored stones occur very rarely in Ceylon deposits, and partially red.