Ruby is a class 04 mineral (oxides), according to Strunz’s classification. It is a reddish-coloured gem; if it is purple, it is called balaj. It owes its colour to the iron and chrome metals with which this variety of corundum (aluminium oxide Al2O3) is associated. Its name comes from ruber, which means ‘red’ in Latin.
It belongs to the corundum family, along with sapphire, a convention of name being the only difference between the two gems. Red corundum is called ruby and sapphire is called all other colors, including pink.
It corresponds to the hardness 9 on the Mohs scale. It is considered one of the four precious gems along with sapphire, emerald and diamond.
Ruby deposits can be found in Burma, Sri Lanka, India, Madagascar, Thailand, Brazil, Colombia, China and Russia, as well as in smaller quantities in South Africa, Australia, Greenland and the United States. Those in the Malaysian Peninsula and Tanzania are very valuable, reaching the value of diamonds of the same size.
The synthetic ruby
Synthetic rubies were first produced in 1923 by mixing alum and chrome pigments. This method allows the production of stones that are chemically and physically very similar to natural ones. However, synthetic rubies are used more in watchmaking than in jewelry.
It is prepared in the following way: First, aluminum oxide (Al2O3) and chromium oxide (CrO3) are used stoichiometrically in a platinum crucible. Then it is placed in a muffle furnace at about 2000 K for about 250 minutes. Finally, it is cooled and the glass is observed to remain on the platinum base.
Ruby is hard (Mohs hardness: 9), and its colour varies with the concentration of trivalent chromium (Cr2O3) in the ore. Due to traces of iron, the colour becomes browner. Its melting point is > 2000 °C, and varies only a few degrees, depending on the concentration of impurities. The relative density is 3.9 to 4.1, depending on the crystalline defects present. Ruby has a trigonal crystalline configuration, and often forms hexagonal crystals.
The ruby is corrosion resistant as it is kinetically and thermally stable as well. It cannot be dissolved in halogenic acids, but it can be dissolved in sulphuric acid to form an aluminium acid sulphate with chromium and aluminium oxide. It is also attacked by perchloric acid, hydrofluoric acid and hexafluorosilic acid.
It is obtained from bauxite mines by the leaching and flotation method. It is also found in the shallow rivers of the Mesolithic zone. Nowadays it is mostly artificially manufactured.
Their uses are not restricted to jewellery, where their value depends on their colour, size, density and purity.they have been widely used in the manufacture of mechanical watches, where they serve as a support or hub for shafts subjected to rotation as their hardness resists frictional wear.
They also have important industrial applications for creating helium-ruby and pure ruby lasers.
They are used to make bracelets and luxury items.
The ancients engraved little in ruby and Pliny the Elder gives as a reason that the seals made with this substance took the wax. The excessive hardness of the ruby, its high price, the rarity of the planes proper for engraving are certainly the real causes that prevented the artists of antiquity from engraving the ruby. It should be noted, moreover, that the impossibility for the ancients to polish the cavities made with this substance could cause the defect of which Pliny speaks.
In the Museum of Odescalque you can find the design of a ruby engraving representing Ceres standing with a spike in his hand. Another ruby engraving features the head of a man with a long beard and frizzy hair who is believed to represent a Greek philosopher. This ruby was carved in the shape of a heart and was part of the collection of the Duke of Orleans.