Ruby

Unlike many gems, ruby is only ever one colour – a pure red.  The reason for this is that ruby and sapphire are varieties of the same mineral, corundum or Al2O3 - only red corundum is called ruby while all other colour varieties are sapphires.  Therefore, ruby has the same gemmological properties as its sister gem, sapphire. 

Pure corundum is colourless with impurities of trace elements causing colour – in the case of ruby, colour is created by trace amounts of chromium and iron with slightly orangey, purplish, brownish or pinkish red caused by relative amounts of these chromophores.  Most sources state that ruby should be of medium to medium dark tone with stones of lighter tone being labelled pink or fancy sapphire
 

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