Garnet is actually a GROUP of silicate materials of differing colours and although generally thought of as red in colour, are known to exist in all colours.
Measuring 6.5 to 7.5 on the Mohs hardness scale, some of the garnets exhibit colour changing properties in different lights - particularly the very rare blue garnet which is due to the high amounts of vanadium in the mineral. In order to identify a garnet, a strong neodymium magnet can be used as this will show a slight movement and magnetic susceptibility.
Garnet actually comes in several different types depending upon the other trace elements included - and the rarest is the Blue Garnet found in Madagascar.
Pyrope (Magnesium/Aluminium Silicate) - Deep red to black colour
Almandine (Iron/Aluminium Silicate) - the most common and typically red Garnet
Spessartine (Manganese/Aluminium Silicate) - Orange to yellow in colour
Andradite (Calcium/Iron Silicate) - Yellow, Brown, Green or Black
Grossular(Calcium/Aluminium Silicate) - Brown, Red, Yellow and Green
Uvarovite(Calcium/Chromium Silicate) - Bright Green
Some examples are shown below