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Garnet

Garnets, traditionally considered ‘semi-precious’, are a large group of minerals containing two isomorphic series. The aluminium and calcium series share a common structure with each variety having slightly different chemical compositions. 

Garnet - semi-precious - birthstone - january
The Garnet family

Garnets are rarely ‘pure’ varieties and are more often combinations containing mixtures of the interchangeable chemical elements. This results in slightly different physical and optical properties among the numerous different varieties.

Most people are familiar with the red pyrope and almandine garnets while few people know that garnets also come in rich green (tsavorite), vibrant yellowish-green (demantoid), vivid orange (spessartine ‘mandarin’), brownish orange (hessonite), and numerous shades of pink, purple, and yellow. With the exception of blue garnet can be found in virtually all colours.

 

Where can you find the garnets you need?

Pierres de Charme has a vast selection of fine quality garnets of all colours. Tsavorite garnets are one of our favourites and we carry many well-cut stones of different shapes and sizes. Our inventory also has many pinks and reds as well as calibrated stones.

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Garnet Fact Sheet

All garnets display bright vitreous lustre even though the hardness of the different varieties varies between 6.5 and 7.5 on the Mohs scale. With the exception of demantoid, which is slightly fragile, garnets are very stable and resistant gems suitable for all types of jewellery. Different garnet varieties can display moderate to very strong dispersion however this ‘fire’ is often masked by the gem’s body colour. Garnets can occasionally display optical phenomena including cat’s eye (chatoyancy) effect and asterism (4 or 6 rayed star). The rare colour change garnet variety shows a bluish-green colour in daylight and a purplish-red colour in incandescent light.

Fact sheet

Chemical composition  

Aluminium series

Fe3Al2(SiO4)3 - almandine garnet

Mg Al2(SiO4)3 - pyrope garnet

Mn Al2(SiO4)3 - spessartine garnet

 

Calcium series

Ca3Fe2(SiO4)3 - andradite garnet (demantoid, melanite, topazolite)

Ca3Al2(SiO4)3 - grossular garnet (hessonite, tsavorite, hydrogrossular)

Ca3Cr2(SiO4)3 - uvarovite garnet (druzy habit)

Crystalline system 

Cubic

Cleavage

None

Refractive index

1.70 to 1.89 (varies depending on variety)

Birefringence  

None

Specific gravity

3.3 to 4.2 (varies depending on the variety)

Anisotrope  

Isotropic

Gemmologists can use many different instruments to identify garnets. Garnet’s distinctive bright vitreous lustre can be seen with the unaided eye. Many of the red and pink varieties contain small amounts of iron (Fe) in their structure that produces a diagnostic absorption spectrum visible with a spectroscope. Using a Chelsea filter with the green varieties can help gemmologists as tsavorite will appear pink or red, demantoid will appear red and hydrogrossular will appear pinkish. Taking a refractive index will identify some varieties while others have refractive indices that are too high for standard refractometers so gemmologists may use specific gravity measurements to confirm a garnet’s identity.

Inclusions, when present, offer gemmologists a great deal of information about the identity of the variety. The following is a list of typical inclusions found in some of the different garnet varieties.

  • Pyrope garnet: Elongated needle-like inclusions
  • Almandine garnet: Rounded or irregular crystals and short rutile needles
  • Spessartine garnet: Wavy healing fissures with jagged outlines
  • Hessonite garnet: Hazy or oily appearance called ‘scotch in water’ due to numerous crystal inclusions
  • Tsavorite garnet: Healing fissures and crystals
  • Demantoid garnet: Groups of fibrous inclusions of an asbestos mineral known as a ‘horsetail’ inclusion

 

Producing Countries

Garnet deposits are found in Brazil, Mozambique, Madagascar, Afghanistan, Pakistan, and Canada. They occur in metamorphic or igneous rocks and can also be found as water-worn pebbles in alluvial deposits. Garnets crystallize in the cubic system and rough garnet crystals are usually rhombic dodecahedrons (12 faces) or icositetrahedrons (24 faces). Red garnets are often found in close proximity to ruby and spinel deposits.

Are garnets treated?

Garnet’s colour and clarity are not significantly improved by heating and therefore they are not routinely treated. When surface reaching fissures are present they may be filled with a resin treatment but this is not common. Most garnets on the market have not been treated.

Garnets in Jewellery

Garnet’s wide range of colours, bright vitreous lustre, and the fact that it is rarely treated make it a popular choice for jewellery. Its moderate hardness, good stability and lack of cleavage make most garnet varieties suitable for all jewellery styles. Transparent stones are facetted and translucent stones may be cut as cabochons. Very dark material can be used for sculpting intaglios. Garnet is the birthstone for the month of January and the gemstone of the 2nd wedding anniversary.

Value and Quality Criteria

As with most gems, the tone and saturation of the colour are the primary quality factor that determines value. For example, fine quality tsavorite garnet should display a rich, saturated, medium toned green. Stones that are too dark, too light, or show greyish overtones will have lower values.

A gem’s clarity will also have an impact on value. Some varieties of garnet are typically more included than others. Red hued garnets are often very clean with few inclusions while orange garnets often have inclusions that are visible to the unaided eye. As a general rule, most consumers prefer gems with few or no inclusions however demantoid garnet is an interesting exception to this rule. Its characteristic ‘horsetail’ inclusion is much sought after by gem enthusiasts and collectors. Lapidaries will carefully cut the stone in order to prominently display the inclusion.

The gem’s size will have a direct impact on the value especially for tsavorite and demantoid garnets as they are rarely found in sizes over 5 carats. Red varieties such as almandine and pyrope are commonly found in large sizes and even large stones have more accessible prices. The rarity of certain varieties will also factor into pricing.

As with all gems, the quality of the cut is an important value factor. Well-cut stones will display better brilliance and command a premium.

Fun Fact

Demantoid garnet shows a very high dispersion (fire) of 0.057, which is higher than that of diamond 0.044.

 

Gre-2. In Cours de base en gemmologie Gem-A. London, 2010 

Garnet Quality Factors. (n.d.). Retrieved August 06, 2016, from http://www.gia.edu/garnet-quality-factor
 
Garnet Description. (n.d.). Retrieved August 06, 2016, from http://www.gia.edu/garnet-description 
 
Garnet Gemstone | Garnet Stone – GIA. (n.d.). Retrieved August 06, 2016, from http://www.gia.edu/garnet