Gems and stones are formed in rocks and sediments. Each mineral or mineral group has a precise chemical composition, made of the same elements or chemical ingredients. These ingredients are given in its chemical formula- the chemical code that defines the mineral and the elements from which it is made.
In addition to the main elements there may be other elements that do not appear in the chemical composition; these accessory elements may affect the properties of the mineral, such as the colour. For instance, the corundum family, which includes Ruby and Sapphire, has a very simple chemical composition (Al203)- two atoms of aluminum (Al) joined with three atoms of oxygen(O). pure corundum would be colourless, but nature seldom enables pure or perfect gemstones to form, so corundum is usually coloured. Chromium and vanadium are accessory elements that give ruby its red colour, iron and titanium give sapphire its blue colour, and other combinations give rise to the many other colours of sapphire such as green, yellow, and pink.