Ruby

Group: Corundum Mineral class: Oxide Colour red, different shades Colouring substance: chrome, iron Pleochroism: strong: red-yellow, dark red Mohs’ hardness: 9 Density: 3.97 - 4.04 Cleavage: absent Fracture: irregular, conchoidal Crystal system: trigonal, hexagonal crystals Transparency: transparent to opaque Lustre: vitreous lustre to Diamond lustre Fluorescence: strong, often red

Its name comes from the Latin ruber or rubrus, which means ‘red’. In the Middle Ages it took the name Ruby.

Like all the corundums, the Ruby has extraordinary hardness. It also shows a strong pleochroism, that is, depending on the angle of observation, the colour shades can change from purple-red to red-yellow. Very rare are Rubies with inclusions of titanium oxide, which are geometrically oriented in a way that they reflect the light in form of a star. This phenomenon can be optimally appreciated if the Ruby is cut in cabochon shape. If these inclusions are ordered in parallel, the light gets reflected like a line and causes the effect of the cat’s eyes. If the inclusions are distributed without a specific order, they result in a silky lustre in the inside of the stone.

Rubies are generally found amongst layers of crystal calcium-carbonate stone, which have been under relatively high pressure and temperatures. The most important deposits are in Asia (Burma, Sri Lanka, Thailand and Cambodia). Opaque, relatively big and splendidly coloured Rubies can be found in Tanzania. The colour is not the best method for determining the geographical origin of a Ruby. The denomination ‘Burma Ruby’ or ‘Ruby of Siam’ is for determining a colour and not an origin.

Transparent Rubies are cut with many facets (brilliant cut, oval cut), while the semitransparent and opaque (especially those with a star or an cat’s eyes) are cut as a cabochon.

Some imitations are so good, that only an expert would distinguish them from the real stone. The only mineral, which is very similar, is the spinell, although its colour is lighter and less intense than the ruby’s. Other similar gems are topaz, tourmaline, garnet and zircon.