Ruby is an aluminium oxide based mineral where the presence of Chromium causes it to become red in hue.
Prices of rubies are generally determined by colour with the brightest red colours commanding the highest prices. Rubies are also graded like diamonds with the Colour, Clarity and Cut being important. They are rated at 9.0 on the Mohs Hardness scale.
Rubies - like other gemstones can be treated and this will make them look much better and in the case of Rubies, this is no exception - they can be heat treated at about 1800c to improve colour, and then sometimes can be lead-glass filled which improves transparency
See below for some examples of Rubies.