Rubies are red. In fact one of the key features in valuing rubies is just how red, as they range from pink through to the most valuable pigeon blood red. Its name comes from ruber, Latin for red.
Ruby is the red variety of the mineral corundum. Only red corundum is entitled to be called ruby, all other colours being classified as sapphires. The close relationship between the ruby and the sapphire has only been known since the beginning of the 19th century. Up to that time, red garnets or spinels were also thought to be rubies. (That is why the 'Black Ruby' and the 'Timur Ruby', in the British Crown Jewels, were so named, when they are not actually rubies at all, but spinels.)
Rubies of more than 3 carats in size are very rare. So it is no wonder that rubies with hardly any inclusions are so valuable that in good colours and larger sizes they achieve prices greater even those paid for diamonds in the same category.
Some rubies display a certain shine, the so-called 'silk' of the ruby. This phenomenon is caused by very fine needles of rutile. Now and then 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 an optical effect known 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 it is moved. Star rubies are precious rarities. Their value depends on the beauty and attractiveness of the colour and, though only to a lesser extent, on their transparency. Fine star rubies, however, should always display rays which are fully formed all the way to the imaginary horizontal line which runs through the middle of the stone, and the star itself should be situated right in the centre.