Garnet is the most commonly misunderstood entry on any gem chart, because garnet is not one mineral. It is a group of silicates sharing a crystal structure and a general formula while differing entirely in composition and colour.
The principal species are almandine, iron aluminium, deep red to brownish; pyrope, magnesium aluminium, the classic blood-red, and the stone of Bohemian garnet jewellery; spessartine, manganese aluminium, orange to red; grossular, calcium aluminium, which includes the honey-brown hessonite and the green tsavorite; andradite, calcium iron, which includes the green demantoid, a stone with dispersion higher than diamond; and uvarovite, an intense green chromium garnet rarely found in cuttable size. Most natural garnets are mixtures of these end members rather than pure species.
So garnet occurs in every colour but blue, and even that exception failed in the 1990s when colour-change garnets from Madagascar were found that appear blue-green in daylight. The universal association of garnet with red is a fact about which species were common and cheap in Europe, not about the group.
The name derives from Latin granatum, the pomegranate, on the resemblance of small red almandine crystals to the seeds.
Garnet has a substantial industrial life that outweighs its ornamental one by volume: it is a major abrasive, used in sandpaper and in waterjet cutting, because it is hard, sharp-fracturing and non-toxic. Most garnet mined is never seen as a gem.