Myths and legends

What is the difference between corundum and ruby?

The word ruby ​​came into Russian from Latin, rubeus or rubber meaning red, and through a later form of the word rubinus. It is believed that Theophrastus (372-287 BC) gave this name to red corundum.
However, in Russian literature, the use of the term ruby ​​has been firmly established to name red corundums, regardless of brightness, thickness or shades of color. And in Anglo-American literature, only brightly or densely colored varieties of red transparent corundum are traditionally called this way. Corundums of any other shade or color are called colored sapphires. Thus, in order to distinguish a pink sapphire from a pale colored ruby, to determine small differences in thickness, intensity of color or various reasons for similar color, specially developed methods are required, available today only to specialized laboratories, and the boundary between these varieties of corundum will still be quite arbitrary.
As for the color of corundum, it depends on the impurities of elements – chromophores (chrome, iron, etc.), and thus, even a slight change in the chemical composition leads to the appearance of different colors. The mineral ruby ​​is crystalline aluminum oxide (AL2O3). In mineralogical collections one can often observe transparent corundum crystals ending in a ruby ​​“cap”. This “cap” appears because a growing corundum crystal “collects” from its entire volume an impurity, trivalent chromium ions, in an amount of about 2-4%, which turns colorless corundum into a beautiful scarlet ruby. It is chromium that is both the carrier of color – the chromatophore, and the carrier of light – the phosphor.
The most prized are rubies of a pure, thick red color – the so-called color of pigeon blood – with a slight purple tint, which appears when the chemical composition of the stone contains chromium oxide in an amount of up to 1,8 – 2%. Increased violet and the presence of orange tints reduce the cost of the stone.
Sometimes a ruby ​​crystal contains microinclusions of “needles” of rutile (this is a mineral, titanium dioxide), the thickness of which is commensurate with the wavelength of light – they give the stone a soft silky shine and iridescence. With their help, the so-called cat’s eye or asterism effect is created.
The phenomenon of asterism is very interesting from the point of view of gemologists – it is an optical effect observed in processed stones with a spherical or other curved (parabolic, ellipsoidal) surface, the so-called cabochons. This type of cut gets its name from the French word caboshe – nail head. Such stones – star rubies – are called asteria, and they are highly valued.
Asterism in rubies is a very rare phenomenon. It is noted mainly in rubies from deposits in Thailand and Sri Lanka, and, as a rule, represents a six-pointed star.
In terms of hardness, ruby, like corundum in general, is second only to diamond (taking second place among minerals in this parameter). Ruby is chemically resistant and is not affected by any acid. It can withstand large temperature changes and high overheating.
Corundum is found in the form of crystals, shaped like barrels, pyramids, prisms, and plates. Sometimes corundums – for example, in Karelian deposits – are found in the form of original pink crystals and their islands.
The main ruby ​​deposits are located in Burma, Sri Lanka, Thailand, Tanzania, Australia, and India. For example, Burma is still called the “Land of Rubies,” and excursion routes for incoming tourists include an exciting journey along mountain roads from Mandalay to Mogok, where the jewelry market and ruby ​​mines are located. The most famous Russian ruby ​​deposits are Hit-Ostrov in Karelia, as well as the Rai-Iz massif in the Polar Urals. Rubies from the Rai-Iz massif are truly remarkable in their color. Here they find tabular corundum crystals of a thick carmine-red color, as well as bright red crystals in the form of hexagonal prisms. For rubies, a weight of 30-40 carats is already unique. The largest ruby ​​weighing 951 carats was found in India. Since currently mined rubies are relatively rarely large in size, they are therefore rarely used in the manufacture of large jewelry. When purchasing a ruby, you should remember that it changes color from light, so it must be stored in a box or a special box. Ruby is a type of corundum, a stunning gemstone that ranks among the most expensive. In terms of hardness, ruby ​​is second only to diamond. The color of this stone is due to the presence of chromium in it. Corundums of other colors are usually called sapphires. Sometimes rubies can be confused with garnets, zirconium and spinel, as for the latter stone, it is precious, but is valued lower than the others. In Russian times, all red stones were called yakhont. Nowadays, artificial rubies with a rich range of colors from dark pink to rich red and dark red colors (corundum) are especially popular. They differ from natural rubies, first of all, in their even, beautiful color, ideal transparency, absence of damage and inclusions, and also in their large size. Rubies, like sapphires, can have a star appearance. Rutile inclusions running parallel to the crystal faces, when exposed to light, form bundles that intersect at an angle of 60°, creating a six-rayed star. Such rubies are valued much more than opaque ones. According to the mineralogist, ruby ​​is a variety of corundum, colored red. The corundum crystal lattice has a perfect design. It consists of oxygen and aluminum ions. Oxygen ions are arranged in layers in a very dense hexagonal packing, and in the voids remaining between them there are aluminum ions, filling two-thirds of these voids. This mineral structure is considered one of the most dense and perfect. Thanks to this, the properties of precious corundum are similar to those of diamond. When aluminum ions in the crystal lattice are replaced by chromium, the stone acquires the famous and very beautiful color of a candle flame. In red rubies the Cr2O3 content is 2%, and in red-black rubies it is approximately 4%. Exposing this gemstone to infrared or ultraviolet radiation “excites” the trivalent chromium ions, charging them with energy, causing the stone to begin to luminesce, emitting light. In this regard, we can conclude that casing rubies glow. The size of this gem should also be mentioned. Large rubies are of greatest value, since they are quite rare. This is evidenced by the period from 1870 to 1970, when more than 300 large diamonds were found weighing more than 200 carats and only a few pure rubies with that weight. Optically pure rubies weighing over 30 carats are very rare and therefore very valuable stones. For example, on the world market the cost of a ruby ​​weighing 2 carats will be higher than that of the same diamond. The larger the size of the gem, the more noticeable the difference in price. In the East, rubies have always been the most valuable stones used in the jewelry industry. Interestingly, before 1800, the word “ruby” was used to describe other red stones. Burmese spinel is ruby-bale, garnets mined in South Africa are Cape rubies, Brazilian pink topazes are Brazilian rubies, garnets from Arizona and Colorado in the USA are Arizona or Colorado rubies, and rubellites from Siberia are Siberian rubies. Ruby owes its rich red color to the admixture of chromium ions, the content of which in densely colored crystals, as mentioned above, can be about 4%. Pink colored ruby ​​stones have nothing to do with rubies, they are corundums or sapphires colored with titanium. The different shades of color also depend on impurities, for example, purple is associated with a high content of vanadium ions, and brown is associated with iron. The color intensity of rubies and their shades can vary greatly. The finest Burmese rubies are often deep blood red in color and are found exclusively in Burma. The color range of Ceylon rubies ranges from light to violet-red, while Siamese rubies range from violet to brown-red. Of particular value are red rubies with a slight purple tint. The presence of orange and increased violet shades negatively affect the cost of the stone. Rubies with a brown tint are the cheapest. As for the color intensity, it also affects the final cost of the gemstone. Rubies of medium-dark tones are most valued, followed by light and dark tones. The price of the stone can be significantly reduced by uneven coloring. The most important thing about a ruby ​​is not its brilliance, but the richness and depth of color, which is why faceted cutting is often used in jewelry when processing rubies. To reveal the beautiful violet-red color, rubies are oriented perpendicular to the optical axis during the cutting process. In addition to color, the cost of a stone is to some extent influenced by various damages, both on the surface of the stone and inside it (cracks or clouding), and, of course, the quality of the cut. So-called cabochons (stones with a smooth and convex surface) are made from star rubies or cat’s eyes; these stones are usually not transparent in nature. The transparency of the stone and the contrast of the star on its surface may indicate the artificial origin of the sapphire or ruby. Ruby deposits located in Indian Burma belong to skarn marbles, which are in contact with dikes of granites or granite-permanites (frozen hot magma of extinct volcanoes). The oldest gem mining area is the Mogok region of Burma, which covers an area of ​​approximately 400 km2. There, rubies appear as granular, nest-shaped clusters or individual crystals embedded in marble. Rubies are mined mainly from placers that were formed many years ago during the destruction of ruby-bearing marbles. The rubies mined there are quite large in size and barrel-shaped or prismatic in shape. The color in most cases is uneven pink or red. Star rubies are often found. As for diagnosing natural corundums, this procedure is difficult because there are many minerals that are very similar to them. The main difference between real ruby ​​and these minerals is its high hardness. Largely because of the hardness of the processing and cutting of ruby, small cracks form on its near-surface part, which is called the “fire marks” defect. It is very difficult to confuse a noble ruby ​​with its synthetic counterpart. The diagnostic procedure is based on the inclusions and physical properties of the gemstone.

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