If ruby is the king of gems, then sapphire is the equally as regal, fabulous cousin that reigns in another country! Both ruby and sapphire come from the same family (variety): corundum. A crystal form of aluminium oxide. The colour is determined by trace elements of vanadium, chromium, iron and titanium.
The poem, “Rubies are red, sapphires are blue”, is almost correct. As red corundum is always a ruby. However all other colours are sapphires, which come in an array of colours. From yellow to purple and pink too. The most famous and sought after rubies, are pigeon-blood red. Likewise the most famous and sought after sapphires are Kashmir as well as cornflower blue.
The most important factors considered for desirability are colour and clarity (lack of inclusions). Cut and carat are subjective.
Sapphires can be heat treated to enhance the colour and clarity. Look out for this on the certificate or speak to your jeweller about it.