Garnet is the name for a group of closely related silicate minerals that share one crystal structure. Most people picture deep red, but garnets come in green, orange, yellow, pink, purple and even color-changing forms. Blue is the only color almost absent. Garnet is the January birthstone, and its lack of cleavage and good hardness make it a practical jewelry stone.
What makes a garnet
All garnets share the general formula X3Y2(SiO4)3. The X site holds a larger metal ion such as magnesium, iron, manganese or calcium. The Y site holds a smaller ion such as aluminum, iron or chromium. Swapping different metals into those sites gives the different garnet species. Because the sites accept mixtures, most natural garnets are blends of two or more species rather than pure end members.
Garnets crystallize in the cubic system, often as twelve-sided dodecahedrons or 24-sided trapezohedrons. Being cubic, they are singly refractive and show no pleochroism. Hardness runs from about 6.5 to 7.5, and refractive index and density vary with chemistry: about 1.71 to 1.89 for RI and 3.5 to 4.3 for specific gravity. Garnets have no cleavage, which makes them fairly tough. The crystal systems in gemstones page explains the cubic system.
The two series
Gemologists split the six main species into two series based on which ion fills the X site.
Pyralspite garnets
These contain aluminum in the Y site. The name joins the first letters of the three species.
| Species | Chemistry | Typical colors |
|---|---|---|
| Pyrope | Magnesium aluminum | Deep red, purplish red |
| Almandine | Iron aluminum | Dark red to purplish red |
| Spessartite | Manganese aluminum | Orange to reddish orange |
Pyrope and almandine mix freely, and rhodolite is a popular purplish red to raspberry pink blend of the two. Pyrope and spessartite also mix, producing malaya garnet and some color-change garnets. See the almandine garnet and spessartite garnet profiles.
Ugrandite garnets
These contain calcium in the X site.
| Species | Chemistry | Typical colors |
|---|---|---|
| Uvarovite | Calcium chromium | Emerald green, small crystals |
| Grossular | Calcium aluminum | Colorless, green, yellow, orange, pink |
| Andradite | Calcium iron | Green, yellow, brown, black |
Grossular includes green tsavorite and orange-brown hessonite. Andradite includes demantoid, the most dispersive garnet. Uvarovite rarely forms crystals large enough to facet. Each has its own page: grossular, andradite and uvarovite.
Colors and causes
Garnet colors come from the metals in their structure. Iron gives the reds and browns of almandine and the yellow to green of andradite. Manganese gives spessartite its orange. Chromium and vanadium give green to tsavorite, demantoid and uvarovite. Some pyrope-spessartite garnets contain vanadium and chromium and change from blue-green or gray in daylight to purple or red under incandescent light. The color in gemstones page covers these chromophores.
Most red garnet is dark, so cutters often keep stones shallow, and older cutters sometimes hollowed out cabochons from beneath to let more light through. A red garnet cut as a cabochon was traditionally called a carbuncle. Star garnets, with four- or six-rayed stars from aligned needle inclusions, occur in some almandine from Idaho and India.
How and where garnets form
Most garnets form in metamorphic rocks, where heat and pressure recrystallize existing minerals. Schists and gneisses commonly carry almandine. Grossular and andradite form in skarns, where hot fluids react with limestone. Pyrope forms deep in the mantle and is brought up in kimberlite, where diamond prospectors use it as an indicator mineral. Spessartite grows in granite pegmatites. The metamorphic rocks and gemstones page explains these settings.
Gem garnet sources include India, Sri Lanka, Madagascar, Tanzania, Kenya, Mozambique, Namibia, Brazil, Russia, Pakistan and the United States. Because garnets are dense and hard, they also concentrate in river gravels along with other heavy minerals.
Bohemian garnet jewelry
The garnet fields of northern Bohemia, now in the Czech Republic, produced small, deep red pyrope garnets for centuries. In the nineteenth century, Bohemian garnet jewelry became fashionable across Europe. Workshops set dozens or hundreds of small rose-cut pyropes closely together in gold or gilded metal to form brooches, necklaces and hair combs. Victorian buyers loved it, and antique pieces are common in estate jewelry today. The Victorian jewelry page covers the period.
Much earlier, garnets were set in gold cells in the cloisonne work of the Anglo-Saxons and Merovingians. The finds from Sutton Hoo show this style, and it is described on the medieval jewelry page.
Synthetic garnet and imitations
Natural garnet species are not grown commercially as gem synthetics. Synthetic garnet-structured materials such as YAG (yttrium aluminum garnet) and GGG (gadolinium gallium garnet) were developed for lasers and industry and were once used as diamond simulants. They are not natural garnet.
Garnet-topped doublets, a thin slice of red garnet fused onto colored glass, were a common imitation for many gems in the nineteenth and early twentieth centuries. A red ring around the girdle in a green or blue stone can reveal one.
Wear and care
Garnets suit everyday jewelry. Their hardness of 6.5 to 7.5 and lack of cleavage mean they handle daily wear well, though softer species like demantoid need a little more care in rings. Garnets are rarely treated. Clean them with warm soapy water and a soft brush. Ultrasonic cleaning is usually safe for untreated stones without fractures, and steam is best avoided for stones with inclusions. The jewelry cleaning guide has details.
Birthstone and folklore
Garnet is the January birthstone on the birthstones by month list. Its name likely comes from the Latin granatum, linked to the pomegranate, whose seeds resemble small red garnet crystals. Travelers in medieval Europe were said to carry garnets for protection on journeys, and the stone has been tied to loyalty and friendship. These are traditions, gathered on the gemstone meanings and folklore page. Garnet is also a traditional gift for the second wedding anniversary in some lists.
Frequently asked questions
Are all garnets red?
No. Garnets occur in red, orange, yellow, green, pink, purple, brown and black. Red almandine and pyrope are simply the most common.
What is the rarest garnet?
Fine demantoid and large tsavorite are among the rarest and most valued gem garnets. Color-change garnets with strong shifts are also scarce.
Is garnet a good everyday stone?
Most garnets are durable enough for daily wear. They have no cleavage and rate about 6.5 to 7.5 on the Mohs scale.
Why is garnet a group and not one mineral?
Garnet covers several species with the same crystal structure but different metal ions. Natural stones are usually mixtures of these species.