Ruby is the red variety of the mineral corundum, one of the hardest minerals on Earth, of which sapphire is also a variety.
Pure corundum is colourless. Slight traces of elements such as chrome, iron, titanium or vanadium are responsible for the colour.
These gemstones have excellent hardness. On the Mohs scale their score of 9 is second only to that of the diamond.
Only red corundum is entitled to be called ruby, all other colours being classified as sapphires. With rubies, the colour is everything (Burmese rubies being the best red colour), the clarity is secondary to colour.The most common colour of sapphire is blue which gets its colour from iron and titanium.
Ruby and sapphire take a brilliant and lasting polish which adds a glittering lustre to the colours.
Rubies are found in the Mogok area of Myanmar (was Burma), Kenya, Tanzania and Zimbabwe. Sometimes rubies are found in marble but most often in river gravels.
Some rubies display a wonderful silky shine, the so-called 'silk' of the ruby. This phenomenon is caused by very fine needles of rutile. Occasionally, one of the rare star rubies is found. Here too, the mineral rutile is involved: having formed a star-shaped deposit within the ruby, it causes a captivating light effect known by the experts as asterism. If rubies of this kind are cut as half-dome shaped cabochons, the result is a six-spoked star which seems to glide magically across the surface of the stone when the latter is moved. Star rubies are precious rarities.
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