Sapphire & Ruby are both from a family of stone known as 'Corundum'. It falls at 9 on MOH's scale of hardness, which makes Sapphires and Rubies a practical gemstone for jewellery. Both can come in lovely variations in colour, for instance Sapphires can be Pink, Blue or Purple. Because this variety of gemstone is available in so many colours it makes an excellent choice for Engagement rings.
Sapphire and Ruby can both be manufactured artificially, that is in a Laboratory. These stones can be referred to as 'Synthetic', they are Corundum so the same chemical composition as the natural stone. The most common method of production is 'Verneuil', to the untrained eye these can look convincingly like natural stones. In the above image the Sapphire is Synthetic, but the Rubies are natural.
Because Sapphires and Rubies are 9 on Mohs scale of hardness and due to their crystal structure, they are a practical choice for jewellery. However it should be noted that Corundum is 7 times harder than Topaz (8 on Mohs scale) and that Diamond is 140 times harder than Sapphire (or Ruby).
Sapphires can come in any colour including Blue, Yellow, Green, Colourless and anything inbetween! Some sapphires can be 'bi-colour' meaning that there are two colours visible within the stone.
People often ask me what constitutes a valuable Sapphire or Ruby? The more desirable stones have a vivid strong colour and excellent clarity. Ruby is the name given to any Red variety of Corundum, and is usually dearer than its counterpart Sapphire, which is the term for any other colour of Corundum. Natural colour stones command a premium and most on the market are heat treated to some extent to enhance the colour.
Go for a stone that is a colour you like and that is clean to the naked eye, by this I mean that you cannot see any inclusions. These will hinder the passage of light through the stone and once you spot it, you can't forget it is there. A larger stone of vivid saturation is more likely to hold its value against something included, cloudy or overly pale/dark.