Garnet is often thought of as red, but these gemstones come in almost any color and are popular for jewelry of all kinds. If you want to buy this January birthstone, that’s great news. In the gem world, the garnet family is one of the most interesting. It’s not a single species, but rather a group of different species and varieties that make it up.
|Varieties||Almandine Garnet, Almandine-Pyrope, Rhodolite Garnet, Andradite, Demantoid Garnet, Melanite, Topazolite, Andradite-Grossular, Mali Garnet, Color Change Garnet, Gadolinium Gallium Garnet, Grossular Garnet, Hessonite, Hibschite, Hydrogrossular Garnet, Transvaal Jade, Tsavorite Garnet, Malaia Garnet (Malaya Garnet), Proteus, Pyrope Garnet, Chrome Pyrope, Pyrope-Spessartine, Umbalite, Spessartite Garnet, Mandarin Garnet, Uvarovite Garnet, Yttrium Aluminium Garnet|
|Colors||A very wide range. See “Garnet Varieties” and “Identifying Characteristics” below and listings for specific garnet species and varieties.|
|Formula||A3B2Si3O12. A = Fe, Ca, Mn, Mg. B = Al, Fe, Ti, Cr. See “Identifying Characteristics” below.|
|Etymology||From the Latin granatus for “grain.” Many garnet deposits are small grains of red crystals in or on their host rock.|
|Crystallography||Isometric. Trapezohedron and dodecahedron forms are common. Cube and octahedron forms are extremely rare.|
|Refractive Index||Varies by species, 1.72-1.95. See Gem Listings for specific varieties.|
|Dispersion||Varies by species, 0.014-0.057. See Gem Listings for specific varieties.|
|Luminescence||Most varieties are inert. Grossulars can show a variety of fluorescence.|
|Absorption Spectrum||See Gem Listings for specific varieties.|
|Optics||Isotropic. Some garnets may show anomalous birefringence. See “Identifying Characteristics” below.|
|Luster||Vitreous, inclining to resinous in grossular, andradite, and some almandines.|
|Specific Gravity||3.40-4.30 (See Gem Listings for specific varieties).|
|Transparency||Transparent to opaque|
Almandines are the most common gemstone in the family of garnets. They come in a wide range of colors, and they are the most common in the family. The combination of almandine and pyrope is the dark red variety that people think of when they think of garnets.
There are very few andradites in the world, and they are one of the rarest. They have the most dispersion out of all garnets, with even more fire than diamond. Demantoids, a type of andradite, are very valuable.
It is very rare for grossulars to be red or dark in color. However, they come in every color, even white, except blue. Their light to medium tones and bright colors make them great for making jewelry. Tsavorites have an emerald-like green color and can be sold for a lot of money. Hessonite “cinnamon stones,” on the other hand, are very popular and cheap.
It isn’t possible to see through hydrogrossulars, which are usually blueish green in color. They can also be pink, white, or gray in color. The classification of hydrogrossular as a garnet species is exciting.
There is a red color in chrome pyropes that can match rubies. There are some dark stones, though.
Spessartites, also known as spessartines, are a type of garnet that isn’t very common. They come in a lot of different shades of orange. Mandarin garnets, a type of spessartite with a bright orange color, are very popular.
Uvarovites are the rarest member of the garnet family. They have a dark, rich green that looks like an emerald, and they are very rare. Facetable material is even rarer and always small.
Non-Gem Garnet Species
Non-gem garnet species include goldmanite, henritermierite, kimzeyite, majorite, schorlomite, and yamatoite. These garnets might be of interest to people who collect rare mineral specimens.
Some people assume these blends are types of garnet, not sub-types of the above species.
Rhodolites are a mix of pyrope and almandine that have a purple color.
Malaya or Malaia Garnet
In the past, this term was used to describe garnets that didn’t fit into any other categories. Now, gemologists know that malaya or malaia garnets are a mix of pyrope and spessartite, and they call them that.
Color Change Garnet
Color shift garnets, which turn blue when exposed to artificial light, have been identified in recent decades. Blue garnets with purple flashes under incandescent light were discovered in Madagascar in the late 1990s. A pyrope-spessartite mixture makes up these color-changing stones. A dramatic color shift from red to purplish red can be seen in some Idaho garnets. These are a mixture of almandine and pyrope.
The following species are known to blend:
Andradite-grossular (also known as grandites or Mali garnets)
Garnet identification is difficult. Several new blends have been discovered in East Africa in the last fifty years. There’s no reason to suppose that all of the conceivable combinations have been found. We don’t know where gemologists will go in the future.
Garnets have a lot in common at the molecular level, despite their differences. For those who aren’t interested in science, here’s a visualization that, while not scientifically sound, can assist illustrate the point. If your hand were a model of a garnet molecule, the arrangement of atoms represented by the palm would be shared by all garnets.
The atoms represented by your fingers, on the other hand, are interchangeable. To put it another way, various atoms can live on your fingers while the palm stays the same. Any time the chemistry is altered, a new species emerges. Even if the structure and related features of a finger stay the same, changing the atoms produces a distinct species (or very nearly so).
The chemistry of garnets, of course, varies significantly more than the hand model can fully reflect. The chemical composition of the gem garnet species is shown below.
- Almandine: Fe3Al2Si3O12
- Andradite: Ca3Fe2Si3O12
- Grossular: Ca3Al2Si3O12
- Hydrogrossular: Ca3Al2(SiO4)3-x(OH)4x
- Pyrope: Mg3Al2Si3O12
- Spessartite: Mn3Al2Si3O12
- Uvarovite: Ca3Cr2Si3O12
As you can see, chemistry comes in a variety of forms. Despite this, they all have a similar core structure. In the isometric system, garnets crystallize. The trapezohedron and dodecahedron are the most popular shapes. Surprisingly, they are rarely found in the shapes of cubes or octahedrons, which are the most prevalent shapes of other isometric minerals. Garnets come in a variety of sizes and shapes, including enormous, granular, and tumbling pebbles.
Rhodolite has been described as one part almandine and two parts pyrope by gemologists for decades. Garnets, on the other hand, are not so straightforward. Rhodolite stones, like all other garnets, contain some of the other species as well.
Garnets are never as simple as having two elements, even if they are present in trace proportions. Furthermore, a solid-state series such as an almandine-pyrope blend does not imply that it is a mixture, Fe3Al2Si3O12 and Mg3Al2Si3O122. Instead, it denotes the presence of both Fe and Mg in the structure.
Are There Any Pure Garnets?
Garnets do not originate as a single pure species in nature. Roughly 83% pyrope, 15% almandine, and about 2% different garnets were found in the purest gem-quality pyrope ever discovered. Almandine and grossular are in the same boat. You’ll find that 80% is about as pure as it gets. Andradite and spessartite garnets, on the other hand, have been discovered to be up to 95% pure. There have been discovered non-gem-sized, colorless garnets that are 97% pure pyrope. It’s something to inspire garnet (or purity) lovers.
The almandine-pyrope and pyrope-spessartite series of garnets were once thought to constitute a straight-line sequence. However, this is insufficient to explain the increasingly complex mixtures we see.
A two-dimensional graph with almandine, pyrope, and spessartite marking the three corners is a more helpful description.
A three-dimensional model is optimal for total accuracy. Andradite, grossular, and uvarovite can all be added to the formula in this way.
Grossular and andradite are found in practically all garnet blends, however they are less prevalent. This graph can be used to show how much of each of these species is present in a single specimen.
Understanding garnet mixes is crucial for gemologists. The mixes of today’s garnets show a great deal of variation. Unless it’s a popular variant, garnets are usually named after their two principal species for identification purposes. Simply put, garnets are not simple two-species minerals.
Gemologists used to classify garnets based on their chemical composition. This terminology is still in use. Pyralspites are garnets that have Al (aluminum) in the B position in their chemical formula (for pyrope, almandine, and spessartite). Ugrandites are garnets having Ca (calcium) in the A position (uvarovite, grossular, and andradite).