Garnet Group

Crystal system: Cubic
Chemical formula/composition: A group of related silicate minerals.  They have a similar crystal structure, and some members are part of an isomorphic series.  Isomorphic meaning same arrangement of atoms, but different chemical elements, for example, almandine (Fe3Al2(SiO4)3) and pyrope (Mg3Al2(SiO4)3)  form such a series.  In this case, iron, Fe, can substitute for Mg, magnesium, and the intermediate amounts of these elements create intermediate composition minerals between the almandine and pyrope end members
Crystal habit: Occurs mainly in dodecahedral forms, well-formed crystals are common. Dodecahedron and trapezohedron
Hardness:  Variable dependent on species, 6.5-7.5.  Some members of the garnet group are a little soft to wear in rings. Almandine (7-7.5), Pyrope (7-7.5), Andradite (6.5-7),  Grossular (7-7.5),  Spessartine (7-7.5), Uvarovite (7.5). 
Specific gravity:
Luster: vitreous
Toughness:  good
Cleavage:  None
Optics: Isotropic
Color: Many colors, mostly green, orange, red, and brown.  Many different shades
Other: U.V. fluorescence: commonly nonfluorescent
Varieties: Several gem varieties, most species have gemstone possibilities.
Localities: Worldwide
Common simulants  None, but colored glass may be falsely substituted.  Spinel has very similar properties.
Synthetics: Synthetics are known but rare.  Synthetics such as GGG and YAG were first made for high tech fields, but were found to be good diamond simulants.  Green YAG or GGG might be used to substitute for demantoid garnet.

The garnet group is one of the most chemically diverse, and garnets are found in many different types of rocks, most notably metamorphic, but also in igneous rocks such a granite and kimberlite.  Gems are produced from almost every variety, and come in a diverse number of  highly attractive colors. The color most often associated with garnet is red, but they occur in many colors, green, orange, purple, brown and are found in many shades of these colors.  Good gem quality crystals can be found that are relatively inclusion free.  Though garnets of 10 pounds or more in weight have been found with frequency, usually gem quality stones are not very large.  Star garnets with 4 rayed stars and color-changing garnets similar in behavior to alexandrite are known, but are rare.

Garnet has excellent luster and nice color, and certain species such as demantoid and Tsavorite are in high demand.

Types of garnet

1)  Pyrope.  Bohemian garnets (pyrope) are perhaps the most frequent found in jewelry.  Pyrope is usually a strong wine red, but can be purple to brown.  Some contains chromium (Cr) such as "cape ruby" found in kimberlites; Cape referring to Cape Town, South Africa.   Chromium (Cr-rich) pyropes are an important indicator mineral in prospecting for kimberlite pipes that may contain diamonds .  The garnets are much more abundant than diamonds in kimberlite.  Sometimes, trails or Cr-rich pyrope in stream deposits, like bread crumbs in a forest, can be followed back to their source, a rock that may contain diamonds.

2) Almandine.  Almandine, the most common garnet found, is at best a crimson red but often is very dark red to brown/black.  It is found in metamorphic rocks all over the world.  Excellent crystals of large size grace museums, but as a gem it is not the most attractive, often too dark, it is commonly cut using a hollow cabochon that decreases the darkness by thinning the gem.

3)  Andradite.  There are several types of andradite.  The most important is the near emerald-green demantoid.  Demantoid is the most valuable species of garnet, because it has properties similar to diamond.  It has a dispersion similar to diamond, which means it splits light into many colors, creating fire.  Demantoid often contains inclusions of fibrous asbestos.  These fibers often resemble a horse's tail and are referred to as horsetail inclusions.  The inclusions, if not too abundant, are desirable and are looked for by collectors.  The demantoid garnet is based on rich green color and grades into the less valuable yellow-green topazolite variety.  Andradite can also be found in less desirable shades such a black and colorless varieties rarely used in jewelry.

andradite garnet

                  Andradite garnets of various colors

4)  Grossular.  Grossular is quite variable in color and can be very pleasing in transparent crystals and gems.  Colors range from green to brownish orange, and pink .  Colorless material is also known.  Two very important varieties are Tsavorite and Hessonite.  Tsavorite is a bright well-saturated green  colored by chromium and/or vanadium.  Hessonite is sometimes called the cinnamon stone; it is colored red-brown to brownish orange.  Though somewhat less valuable than tsavorite, both these types are important in jewelry.

5)  Spessartine.  Spessartine is most important as gem in its orange color.  It varies from orange yellow to reddish orange.

6) Uvarovite. Uvarovite typically occurs as very fine crystals.  It is invariably a rich green, often emerald green in color.  Chromium is responsible for this.  Uvarovite is so fine grained, that gems are never cut from it, but the attractive green crystals occur in clusters or mats of crystals in rock, and these cluster can be cut to form pieces used in jewelry.  The folksy effect of the many small crystals can be attractive in pendants, brooches, etc.

garnetdemantoid garnet

Orange and Wine-red Garnets                               Demantoid  Garnet

Identifying properties of garnet

Garnet crystals are often well formed and usually occur as ball-like dodecahedrons or related forms. 

Because all garnets are isotropic, they are black when placed between two crossed polarizers in a polariscope.  They show no birefringent colors.  Though spinel is also isotropic, it usually forms octahedral crystals and may show a little color between crossed polarizers because of anomolous birefringence.   Garnets in general are not confused with many other stones, although tsavorites might be confused with other green stones such as emerald or tourmaline.  Polarized light would help here.  In general, consider ruby, tourmaline, and fire opal, which can have similar colors.  If further test are needed, using a spectroscope or testing for specific gravity (density) may be helpful. 

Garnet-Topped Doublets

Garnet-topped doublets were used in the past to add a durable top to a piece of colored glass.  The doublet's crown was topped with garnet.  The garnet might be fused to the glass pavilion at a position other than the girdle to avoid detection.  The top of  the garnet was harder than glass and could protect the doublet from scratching.  Since the garnet top is thin, the color of the glass provides most of the stone's color.   Hence garnet topped doublets may be colorless if  the top  is thin.


These synthetic "garnets" have the same structural formula as some natural garnets, but have not been found in nature.  Perhaps  GGG or Yag could confused with natural garnets, but they are also mentioned here because they were important diamond simulants and are still used today by private/amateur cutters even though they have largely been replaced by cubic zirconium as a diamond simulant.

GGG stands for gadolinium gallium garnet and YAG stands for yttrium aluminum garnet.  These two substances have the same formula as natural garnets and the same crystal shape, but are rare earth element garnets that never occur in nature.  They are man made, using either Czocharalski method of crystal pulling or the flux fusion method (discussed under synthetic stones).  They have very high dispersion, similar to diamond, and are very dense because of their rare earth composition.  The only natural garnet they might be confused with is Andradite.

Very large boules or crystals can be grown, so large stones can be produced.  Though these stones are not commonly used in jewelry because of supplementation by cubic zirconium, they are still used in laser applications, etc., and it is possible that you might find them used occasionally as gemstones.  YAG lasers are still commonly used so the material is out there.  Colorless stones with diamondlike or garnetlike properties, but too dense to be diamonds could easily be GGG or YAG.


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