Although garnet is commonly associated with the color red, these gemstones can be found in almost any color and are popular choices for jewelry of all types. That's excellent news if you're in the market for this January birthstone. The garnet family is one of the most complex in the gem world. It's not a single species but rather consists of several species and varieties. This gem listing will focus on qualities common to all garnets and offer a brief overview of their diversity.
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|Varieties||Almandine, Almandine-Pyrope, Rhodolite, Andradite, Demantoid, Melanite, Topazolite, Andradite-Grossular, Mali Garnet, Color Change Garnet, Gadolinium Gallium Garnet, Grossular, Hessonite, Hibschite, Hydrogrossular, Transvaal Jade, Tsavorite, Malaia Garnet (Malaya Garnet), Proteus, Pyrope, Chrome Pyrope, Pyrope-Spessartine, Umbalite, Spessartite, Mandarin Garnet, Uvarovite, Yttrium Aluminium Garnet|
|Crystallography||Isometric. Trapezohedron and dodecahedron forms are common. Cube and octahedron forms extremely rare.|
|Colors||A very wide range. See “Garnet Varieties” and “Identifying Characteristics” below and listings for specific garnet species and varieties.|
|Luster||Vitreous, inclining to resinous in grossular, andradite, and some almandines.|
|Hardness||6.5 – 7.5|
|Transparency||Transparent to opaque|
|Formula||A3B2Si3O12. A = Fe, Ca, Mn, Mg. B = Al, Fe, Ti, Cr. See “Identifying Characteristics” below.|
|Optics||See "Identifying Characteristics" below.|
|Etymology||From the Latin granatus for “grain.”|
Unlike minerals such as beryl or corundum that are a single species with colored varieties created by trace elements, garnets come in different species and are never found in their pure state. They are always mixed with other garnet species. Garnets are called a solid-state series or a blend. Some of these blends have distinct features and are recognized as varieties of garnets in themselves. What makes them all garnets is the same crystal structure and similar properties.
For more information on these species and their own varieties, click on the links to the individual listings.
- Almandine: The most common gemstone in the garnet family comes in a wide range of colors. The blend of almandine-pyrope is the dark red variety popularly associated with garnets.
- Andradite: One of the rarest garnets. These gems have the highest dispersion of all garnets and have even more fire than diamond. Green demantoid is especially prized.
- Grossular: Unlike other garnets, these gems are rarely red or dark in tone. They can be found in every color, even colorless, except blue. Their light to medium tones and vibrant colors make them excellent jewelry stones.
- Hydrogrossular: These garnets are never transparent. They are commonly blueish green in color but are sometimes found in pink, white, and gray. (Editor’s Note: the classification of hydrogrossular as a garnet species is debatable).
- Pyrope: Chrome pyrope garnet is a red that rivals ruby. However, they’re very dark in tone.
- Spessartite: (Also known as spessartine). This somewhat rare garnet comes in a variety of orange colors. Mandarin garnets are highly sought.
- Uvarovite: The rarest stone in the garnet family is a dark, rich green that rivals emerald. Facetable material is always small and extremely rare.
The following blends are considered to be varieties of garnet and not sub-varieties of the above species.
- Rhodolite: a blend of pyrope and almandine with a distinctive purplish color.
- Malaia: a term originally used to describe garnets that didn’t fit into standard categories. It’s now recognized as a blend of pyrope and spessartite.
- Color change garnets: In recent decades, garnets have been discovered that turn blue in artificial light. In 2007, blue garnets that are red with purple flashes under incandescent light were discovered in Madagascar. These color change stones are a pyrope-spessartite blend. Some Idaho garnets show a strong color shift from red to purplish red. These are an almandine-pyrope mix.
The following species are known to blend.
Identifying garnets is complex. In the last fifty years, several new blends have been discovered in East Africa. There’s no reason to believe that all the possible blends have been discovered. We don’t know what the future holds for gemologists.
Garnets share common properties at the molecular level despite considerable variations. For those not scientifically inclined, here’s a visualization that, while not scientifically rigorous, may help illustrate this. If your hand were a model of a garnet molecule, all garnets would share the arrangement of atoms represented by the palm. However, the atoms represented by your fingers are interchangeable. In other words, different atoms can reside on your fingers, while the palm remains the same. Anytime you change the chemistry, you have a different species. Change the atoms of a finger and you have a different species, even though the structure and related properties remain the same (or very nearly so).
Of course, the chemistry of garnets varies far more than the hand model can accurately demonstrate. Below is the chemical composition of the gem garnet species.
- Almandine: Fe3Al2Si3O12
- Andradite: Ca3Fe2Si3O12
- Grossular: Ca3Al2Si3O12
- Hydrogrossular: Ca3Al2(SiO4)3-x(OH)4x
- Pyrope: Mg3Al2Si3O12
- Spessartite: Mn3Al2Si3O12
- Uvarovite: Ca3Cr2Si3O12
As you can see, there are several variations in chemistry. Nevertheless, they all maintain the same basic structure. Garnets crystallize in the isometric system. Their most common forms are the trapezohedron and dodecahedron. Curiously, they rarely appear as cubes or octahedrons, the most common shapes of other isometric minerals. Garnets may also be massive, granular, and in tumbled pebbles.
For decades, rhodolite has been described as one part almandine and two parts pyrope in gemology education. However, garnets are not that simple. Rhodolite gems, as well as all other garnets, have some of the other species in the mix as well. These may be present in very small amounts, but garnets are never as simple as just two ingredients. Furthermore, a solid-state series like an almandine-pyrope blend doesn’t mean that it’s a mix of two kinds of molecules, Fe3Al2Si3O12 and Mg3Al2Si3O12. Instead, it means that the structure contains both Fe and Mg.
Garnets are not found in their pure form in nature. The purest gem-quality pyrope ever discovered contained about 83% pyrope, 15% almandine, and about 2% other garnets. The same is true for almandine and grossular. 80% is about the purest you’ll encounter. However, andradite and spessartite garnets have been found as high as 95% pure. Non-gem sized, colorless garnets of 97% pure pyrope have been found. It’s something to give the lover of garnets (or purity) hope.
Garnets were traditionally described as a straight-line series, such as almandine-pyrope or pyrope-spessartite. This is not accurate enough to explain the complex blends we now see.
A more useful description is a two-dimensional graph, with almandine, pyrope, and spessartite marking the three corners. A gem’s chemistry would rarely be located on one of the flat sides. In actuality, it would be a point inside the triangle, indicating how much of each element is present.
For complete accuracy, a three-dimensional, five-pointed model is used. This adds andradite and grossular to the formula. While less common, these are found in almost all garnet blends. With this graph, one can indicate how much of each of these species is present in an individual specimen.
It’s important for gemologists to understand garnet blends. The garnets known today show considerable varieties in their mixtures. For identification purposes, standard practice is to name garnets by their two primary species unless it’s a common variety. Just understand that garnets aren’t simple, two-species minerals.
In the past, garnets have been grouped according to chemical composition and are still referred to in this manner. Garnets that contain Al (aluminum) in the B position in their chemical formula are known as pyralspites (for pyrope, almandine, and spessartite). Garnets with Ca (calcium) in the A position are known as ugrandites (uvarovite, grossular, and andradite).
These properties are very dependent on chemistry. Pyrope, almandine, and spessartite are generally isotropic. However, the presence of the large Ca (calcium) atom in uvarovite, grossular, and andradite makes them birefringent. This may be due to strain but more probably has a structural explanation. Grossular and andradite are almost always zoned, often twinned, and distinctly not isotropic under the microscope.
Due to their tremendous range of overlapping colors, garnets can’t be identified on the basis of color alone. This information is for reference only.
- Uvarovite: dark green.
- Grossular: colorless, white, gray, yellow, yellowish green, green (various shades: pale apple green, medium apple green, emerald green, dark green), brown, pink, reddish, black.
- Andradite: yellow-green, green, greenish brown, orangey yellow, brown, grayish black, black. The color is related to the content of Ti and Mn. If there’s little of either element, the color is light and may resemble grossular.
- Pyrope: purplish red, pinkish red, orangey red, crimson, dark red. Note: Pure pyrope would be colorless; the red colors are derived from Fe + Cr.
- Almandine: deep red, brownish red, brownish black, violet red.
- Spessartine: red, reddish orange, orange, yellow-brown, reddish brown, blackish brown.
- Malaia: various shades of orange, red-orange, peach, and pink.
- Rhodolite: usually has a distinctive purplish color.
Synthetic garnets have had an effect on the gem world. Before the advent of cubic zirconia (CZ) in the late 1970s, synthetic garnet was the primary diamond simulant. While they may have a smaller presence in today’s market, you’ll still find these synthetic gems.
YAG, or yttrium aluminium garnet, was the first synthetic garnet available on the jewelry market. In its pure state, YAG is colorless. However, it can be created in almost every color. The dopants used for coloring are also responsible for the wide range of refractive index and specific gravity. Colorless YAG is near the bottom of each property. YAG is not brittle and wears well. However, its dispersion is a bit low for a diamond substitute.
GGG, or gadolinium gallium garnet, has a high dispersion (.038). Gadolinium and gallium are much more expensive than yttrium. However, with a dispersion close to diamond (.044), GGG makes a wonderful substitute. As with YAG, the dopants used for coloring are also responsible for the wide range of refractive index and specific gravity. Colorless GGG is near the bottom of each property. Less expensive CZ has mostly replaced GGG in jewelry today. However, these synthetics are still available in many colors. They cut beautiful gems and are a favorite of many lapidaries.
In Greek mythology, Proteus was a shape-shifting sea god. His name now means someone who easily changes their appearance or principles. Garnets can be changed, but not easily. Proteus are the only treated garnets. A few almandine-pyrope gems from the US will change into Proteus. All the other types resist change. This treatment brings a thin layer of metals to the surface of the stone. This causes it to have a dual appearance. In reflected light, Proteus garnets have a dark gray, metallic luster, much like hematite. In transmitted light, the dark red shows through.
Garnet crystals are usually small, from microscopic up to about 6 inches in the case of grossular. Many deposits are small grains of crystals in or on their host rock. Garnets in rock, with poor external forms, may be much larger, such as the almandine from Gore Mountain, New York, which reaches a diameter of 60 cm. A few spessartites in Brazil have weighed several pounds and have retained great transparency and fine color. However, these are very rare. A typical garnet crystal is about half an inch to an inch in diameter.
Garnets are best cleaned with warm water, detergent, and a soft brush. Although relatively hard and tough, garnets can be heat sensitive. Extreme heat should be avoided. See the individual entries for specific species and varieties for more recommendations. Consult our Gemstone Care Guide and Gemstone Jewelry Cleaning Guide for more information.
by Dr. Joel Arem; Donald Clark, CSM IMG