Many people, when they think of garnet, think of a deep red stone. This colour is usually pyrope / almandine garnet, and is just one of many different types of garnet that occur.
Garnet is a silicate and occurs in two main (and related) series. The first, which contains aluminium, include the deep red pyrope and almandine garnets, and purple rhodolite garnet, as well as the yellowish-orange spessartine garnet. The second series, which contains calcium ions, includes the orange hessonite garnet, green tsavorite, demantoid and uvavorite garnets, and hydrogrossular garnet (which can vary in colour).
Garnets are often clean-looking stones internally, but can show a variety of different inclusions.
Garnet is the birthstone for January.