Garnets are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives.
All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different species are pyrope, almandine, spessartine, grossular (varieties of which are hessonite or cinnamon-stone and tsavorite), uvarovite and andradite. The garnets make up two solid solution series: pyrope-almandine-spessartine (pyralspite), with the composition range [Mg,Fe,Mn]3Al2(SiO4)3; and uvarovite-grossular-andradite (ugrandite), with the composition range Ca3[Cr,Al,Fe]2(SiO4)3.
The word garnet comes from the 14th-century Middle English word gernet, meaning 'dark red'. It is borrowed from Old French grenate from Latin granatus, from granum ('grain, seed'). This is possibly a reference to mela granatum or even pomum granatum ('pomegranate', Punica granatum), a plant whose fruits contain abundant and vivid red seed covers (arils), which are similar in shape, size, and color to some garnet crystals.
Hessonite garnet is also named 'gomed' in Indian literature and is one of the 9 stones in Vedic astrology that compose the Navaratna.
Garnet species are found in every colour, with reddish shades most common. Blue garnets are the rarest and were first reported in the 1990s.
Garnet species' light transmission properties can range from the gemstone-quality transparent specimens to the opaque varieties used for industrial purposes as abrasives. The mineral's luster is categorized as vitreous (glass-like) or resinous (amber-like).
Because the chemical composition of garnet varies, the atomic bonds in some species are stronger than in others. As a result, this mineral group shows a range of hardness on the Mohs scale of about 6.0 to 7.5. The harder species like almandine are often used for abrasive purposes.
Red garnets were the most commonly used gemstones in the Late Antique Roman world, and the Migration Period art of the "barbarian" peoples who took over the territory of the Western Roman Empire. They were especially used inlaid in gold cells in the cloisonné technique, a style often just called garnet cloisonné, found from Anglo-Saxon England, as at Sutton Hoo, to the Black Sea. Thousands of Tamraparniyan gold, silver and red garnet shipments were made in the old world, including to Rome, Greece, the Middle East, Serica and Anglo Saxons; recent findings such as the Staffordshire Hoard and the pendant of the Winfarthing Woman skeleton of Norfolk confirm an established gem trade route with South India and Tamraparni (ancient Sri Lanka), known from antiquity for its production of gemstones.
Pure crystals of garnet are still used as gemstones. The gemstone varieties occur in shades of green, red, yellow, and orange. In the US it is known as the birthstone for January. The garnet family is one of the most complex in the gem world. It is not a single species, but is composed of multiple species and varieties.
Garnet is the birthstone of January. It is also the birthstone of Aquarius and Capricorn in tropical astrology. In Persia this birth gem was considered a talisman from nature's forces like storm and lightning. It was widely accepted that garnet could signal approaching danger by turning pale.