Corundum - knowledge base, jewelry
Properties of corundum, physiological effects, zodiac signs and corundum jewelry.
The most precious gemstone after diamonds.
It is mainly known as ruby and sapphire. The material of corundum is pure crystallised alumina or alumina, with small amounts of chromium and iron as a staining agent. The most common crystal forms of corundum are hexagonal bipyramids and hexagonal columns combined with base and rhombohedron. It is mostly found in solitary, spherical crystals, mainly in second-order lying among pebbles and sands; it is derived from older rocks, notably granite, syenite, gneiss and mica shale.
Chemical and physical properties of corundum:
Formula: Al2O3 aluminium - oxide
Crystal system: trigonal, rhombohedron
Color: red, blue, green, orange, pink, colourless, yellow
Transparency: low translucent, opaque
Fracture: conchoidal, splintery
Mohs scale hardeness: 9,0
Specific gravity: 3,97 – 4,05 g/cm³
After diamonds, it is the hardest mineral in the corundum.
There is a wide variety of colours, rarely water clear, more often blue, red, yellow, brown, grey, etc. corundum. They are distinguished by their colour and transparency. The opaque ones are the common corundum or diamond platinum, the transparent, shining red-coloured corundum is the ruby, the blue corundum is the sapphire, the very dirty, granular structure, which is usually mixed with iron compounds, is the emerald.
1. The common corundum or diamond spar
It grows in opaque bluish, dirty blue-grey, greenish or brownish crystals with a rough surface, or in iron, scattered in older rocks.
2. The ruby
The red colour can vary from pale pink to dark carmine red. The larger ones are particularly rare, as 10 carats are a great rarity. A flawless, full-coloured ruby larger than 3 carats is more expensive than a diamond of the same size and quality. When the ruby is heated, it loses its red colour and turns green, but this colour is not permanent because it turns red again when it cools. It is also given different names according to the different shades of colour.
In particular, dawn red tending to yellow is called oriental amethyst or amethyst sapphire. This colour is caused by chromium.
The finest rubies are brought from Burma, where the mines are located in the Mogok Valley, about 120 kilometres north of Mandalay. They work in some 78 mines; the mining process involves digging simple holes, putting the gravel into baskets and picking out the rubies from there. In terms of value, it has been rising recently. A dark-coloured, flawless diamond, if no heavier than a carat, is paid twice the price of a first-quality diamond of equal size.
Many other red-coloured gemstones are sold instead of rubies, including spinel, garnet and zircon, but also roasted amethyst, topaz (Brazilian ruby), which turns red in the heat, tourmaline and even red quartz. More recently, it is also made artificially in large quantities, which are polished and set into jewellery. Ruby is also used in the bearings of fine axles in watchmakers.
3. The sapphire
Its colour can range from very light to the darkest blue. The most beautiful is the pure cornflower colour, called indigo sapphire or hy sapphire, and the pale, slightly off-white no sapphire. The blackish or greenish blue is the lynx sapphire or cat sapphire, the pure greenish blue is the oriental aquamarine, the very pale is the water sapphire, the water clear is the white sapphire (leucosa sapphire).
Sapphire is more common than ruby, and a rich, even colour is not uncommon. The finest are found in the West Indies (Burma, Pegui, Siam) and Ceylon. More recently, very dark coloured, almost black sapphires have been traded from the Bangkok area (Siam).
In addition to the size, the richness of the colour and the flawlessness of the stone enhance its value. Sapphire becomes water clear when gently heated. This is how stains and other imperfections are removed. The largest known sapphire is 951 carats.
Sapphire is also produced artificially, but it can also be forged with blue-coloured cyanite or cordierite.
4. The starry stone or armature
The star stone is a corundum version with a six-pointed star on its convex faces.
5. Oriental emerald
Pure green corundum, the rarest of all gemstones. The yellowish-green corundum is oriental chrysolite, which is less rare; the pure bright yellow corundum is oriental topaz, topaz sapphire or yellow sapphire, not very common.
Emerald is a granular corundum. Its high hardness makes it a very good abrasive. Emery occurs in granular limestone or marble as a mixture of corundum and magnetite.