Never mind the romance of the glowing red gemstone, let's think chemistry. Rubies are made of corundum, a mineral of aluminium oxide. For every 5,000 aluminium atoms in a typical stone, there is one alien. In other words, these jewels are impure. (Forget the virtuous woman and call up the scarlet.) If they hadn't fallen prey to the rogue element, they would be colourless. But they succumbed to chromium, which turned them red. Iron and titanium perform the same trick on corundum except they produce the blue sapphire.
Rubies come in many shades, from deep cochineal through to almost pink. The most prized is the Mogok, or pigeon-blood ruby, found mainly in bands of limestone beside a road in Myanmar - the road to Mandalay, in fact.
Other sources of true ruby include Thailand, Sri Lanka and Afghanistan, but there really isn't much of the stuff around. Would-be buyers (reember, a girl can have more than one best friend) have to be careful. There are also Cape rubies, Australian rubies and Arizona rubies - except that they are garnets. There are Siberian rubies - except that they are red tourmaline. There are synthetic rubies and there is balas ruby - except that that is ruby spinel.
Spinel has the colour of true ruby, but there the similarity ends. It is made of magnesium aluminium oxide and crystallises in a cubic style. True ruby has a hexagonal structure and is the second hardest substance after diamond.
Nevertheless ruby spinel has done pretty well for itself. A 5cm-wide example, one of the world's largest, gleams from a Maltese cross in the front of England's imperial state crown (pictured). The stone was given to Edward, the Black Prince, by Pedro the Cruel of Castile in 1367. Henry V nearly lost it when he wore it at the Battle of Agincourt in 1415 - which might have given the court scribes something to weave a little poetry around. How about: "Who can find a careless man? He's the one over there, looking for his ruby."