Corundum is crystalline Aluminium Oxide (Al2O3) which in its pure form is colourless. It has two varieties, based on colour. The red variety is called Ruby, while Corundum of any other colour should be refered to as Sapphire. 

The presence of an additional ion of Chromium in Ruby absorbs some wavelengths of light and acts as the colouring agent, while the blue colour in Sapphire is due to the presence of Iron and Titanium. Sapphires can be almost every colour of the rainbow, violet stones owe their colour to the addtion of Vanadium for example, while Iron gives rise to yellow and green gems.

There is no clear demarkation point between Rubies with a strong red colour and paler examples, the distinction is made on the basis of colour saturation with paler stones referred to as Pink Sapphire.

Many treatments are applied to Corundum to improve appearance. Standard heat treatment is an accepted enhancement, since it brings out tthe colour naturally inherant within the stone.  Any gem which has undergone a process that adds to or changes the appearance in other ways should be refered to as 'treated', such as those with a diffusion coating or those with lead-glass filling.