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gemstone dictionary

Garnet

Due to the extraordinary complexity of the subject we have given Garnet much more room than most other entries in this dictionary.
The following text deals with the Garnet group in general. See the special entries for information on particular garnet varieties like Almandine, Rhodolite, Spessartite, Tsavorite etc.

Garnet is a complex group of miscible silicates with isomorphous replacement. Isomorphous means, that some elements in the chemical formula can be replaced by others without changes in the crystal structure. Thus the chemical composition of Garnet varies within certain limits.

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Origin of name: from Latin granum = grain or Latin grantum = pomegranate because of the resemblance of some Garnet crystals to the seeds of this fruit.

In a seventeenth century translation of the Bible, Garnets are termed "carbuncles", from Hebrew "bareketh" which derives from the word "barak", hebrew for lightning. It is unclear, however, if this really meant Garnet because at that time more or less all red gemstones were called carbuncles.

History: for the last 2500 years the use of Garnets as jewellery is well documented by burial objects. Garnets were found in Bronze Age graves in Bohemia as well as on Egyptian mummies.

One of the first descriptions of Garnet is to be found in the Book of Exodus, where the gems decorating Aaron´s breastplate are listed.

The first "gemmological" treaty is provided by Theophrastus in the third century B.C. He wrote about a stone called "Anthrax" which he said was incombustible. Later translators and interpreters, amongst them Pliny, concluded from this that Theophrastus meant corundum, that is Ruby and Sapphire. However, Theophrastus also wrote that sealstones were made of Anthrax. Now sealstones made from corundum were hardly ever used because they were very difficult to engrave due their supreme hardness. Garnet sealstones, on the other hand, were quite common and so today it is assumed that Anthrax really was Garnet.
By the way, in ancient times gemstones were much more a privilege of the rich than they are today. According to Theophrastus a sealstone cost up to forty pieces of gold!

About fourhundred years later Gaius Plinius Secundus the Elder wrote his monumental work "Naturalis Historia". In thirtyseven books he covers mathematics, cosmology, botany, anthropology, zoology, geography, medicine and much more. The last four books deal with minerals and gemstones.
Pliny was the first to attempt quality grading. He distinguished male from female Garnets, male Garnets having darker colours and more brilliance. The best stones were described as "those in which the fiery red shade passes at the edge into amethyst-violet". In this respect the Romans were in accordance with modern taste because today Garnets with a distinct violettish tinge are the most sought-after and fetch the highest prices of all red Garnets.

Naturally the high prices called the bad boys to the arena even then and Pliny in some detail addressed falsifications and their identification. One of the oldest methods to improve the visual appearance of gems is to place a metal foil underneath which greatly enhances brilliance. The technique of foil-backing gems is still in use today, mainly to restore old jewellery true to the original.

Somewhat quaint seems Pliny´s recommendation to soak dull stones in vinegar for two weeks which would produce an increase in brilliance that would last for months.

Fake carbuncles were made of glass. Pliny suggests the use of a grindstone for identification. Today this is known as the Scratch Hardness Test and it is still applied, albeit more so in mineralogy than in gemmology, which has to work non-destructively. One such non-destructive test is to measure specific gravity which Pliny anticipated by suggesting to compare the weight of a Garnet under test with that of an undoubtedly genuine Garnet. The specific gravity of glass is significantly lower than that of Garnet. So this test in principle works very well, at least as long as you compare stones of similar size.

Fakes containing inclusions presented a bigger challenge back then. Glass was produced in all colour shades of red Garnet and by heat quenching fractures similar to those in natural gemstones were induced.

Up to the Middle Ages many treaties about gemstones in general and about Garnets in particular were published. It seems though, that the authors mostly just copied Theophrastus and Pliny. Gaius Julius Solinus really is the only author worth mentioning, because he was the first to use the term "pyropus" to describe the "fiery" quality of some Garnets.
Epiphanius of Salamis, by the way, thought that carbuncles were difficult to steal because their inner fire would shine through the thieve´s clothes and thus give him away.

The next author who produced a substantial contribution to Garnet research was Albertus Magnus, the great thirteenth century theologist and natural scientist. He divided red gemstones into three groups: belagius (which later became "Balas" for Spinel), rubinus and granatus. The generic term for all these was, for a long time to come, carbuncle.

In 1546 the German scholar and scientist Georg Agricola published his ground-breaking opus "De Natura Fossilium" in which he first mentioned the Bohemian Garnet occurrences.

Last but not least Anselmus Boëtius de Boodt, Belgian mineralogist and physician, must be mentioned here. In his work "Gemmarum et Lapidum", published in 1609, he dealt with Bohemian Garnets, grading tem into many different categories, according to quality and price.

All early scientists alike were handicapped by the absence of a sound mineralogical foundation. Any attempt to describe and classify gemstones was therefore reduced to the comparison of colour, hardness, crystal shape and, with some limitations, specific gravity. Classification of the exceedingly complex Garnet group in particular posed a major problem.

Modern mineralogy came into being at the end of the eighteenth century. Natural scientists like Haüy, Lavoisier, and Mohs (after whom the scale of hardness was named) were amongst the founding fathers and had a decisive influence in bringing thousands of years of magical and metaphysical speculation to an end. Slowly but surely science began to understand chemical composition, shedding light onto Garnets and all other gemstones.

Modern garnet research: in gemmology one mostly talks about species, e.g. corundum, and variety, e.g. Ruby. With Garnets the matter is more complicated. The term "Garnet" actually refers to the Garnet group which consists of species, the so-called end-members, and their varieties.

Garnet chemistry: thank goodness, A3B2[CO4]3, the basic structural formula is the same for all Garnets. A, B and C being definite places in the crystal lattice, which can be taken by these elements:
aluminium, calcium, chromium, iron, magnesium, oxygen, silicium, titanium, vanadium, manganese and various trace elements.

Contents of these elements, which isomorphically replace each other, vary greatly. The number of possible combinations is limited by the chemically pure end-members with ideal composition, but it still is confusingly large. Furthermore the discovery of ever new varieties keeps complicating matters. End-members in their pure state do not exist in nature. 99% of all Garnets are mixtures of two end-members. They are said to be in "solid solution".

Theoretically as many as sixty end-members are possible. Twenty are known to exist, six of which – Pyrope, Almandine, Spessartite, Grossular, Andradite and Uvarovite are of importance in Garnet classification but only the first five exist in commercial quantities.

The six end-members mentioned above are often referred to as two series of species:

Pyralspite series

Ugrandit series

Pyrope

      Mg3Al2(SiO4)3

Grossular

      Ca3Al2(SiO4)3

Almandine

      Fe3Al2(SiO4)3

Andradite

      Ca3Fe2(SiO4)3

Spessartite

      Mn3Al2(SiO4)3

Uvarovite

      Ca3Cr2(SiO4)3



What complicates Garnet research is the fact, that mixtures of all these end-members are conceivable and in fact do exist. Changes in chemical composition bring about changes in physical properties like refractive index and specific gravity, not to mention colour. No wonder early garnet researchers faced tremendous problems when attempting some kind of classification. Today chemical analysis enables us to identify the elements contained and thus categorize garnets exactly.

Crystallography: Garnet crystallizes in the cubic system. All cubic crystals are singly refractive which means that a ray of light entering the stone is not split into two rays, as is the case in doubly refractive minerals. Other cubic minerals are Diamond and Spinel which is one reason why Garnet and Spinel were thought to be one species (Balas) for centuries.

Physical and optical properties: greatly varying due to the variable chemical composition.

Hardness: 6.5-7.5. Most Garnets used in jewellery have a hardness of +7 thus being (in some cases significantly) harder than e.g. Amethyst.
Attention: some Garnets, notably Demantoid and, to a lesser extent Tsavorite, are quite brittle. Set with care.

Refractive index: singly refractive 1.73 to 1.89

Specific gravity: 3.40-4.30. The S.G. of most Garnets is above that of Diamond (3.52)

Disperison: not very pronounced, with one notable exception. Generally between 0.020 and 0.028 (Diamond 0.044). The one exception is Andradite whose dispersion of 0.057 is abvoe that of Diamond but whose fire is masked by it´s strong body colour.

Colour: Garnets occur in all colours except blue.

Basically there are two causes of colour in gemstones. Both rely on the colouring effect of metal oxides:

1. allochromatic = coloured by a foreign substance
the colouring metal oxides are present as impurities
Examples:
Pyrope (MgAl-Garnet): coloured by iron or chromium
Grossular (CaAl-Garnet): coloured by manganese, iron, chromium and/or vanadium

Chemically pure allochromatic Garnets are colourless. These "Leuco-Garnets" are extremely rare and fetch high prices on the collectors market.

2. idiochromatic = self-coloured
Colouring metal oxides are part of the chemical formula.
Examples:
Almandine (FeAl-Garnet): coloured by iron
Spessartite (MnAl-Garnet): coloured by manganese

Andradite is a CaFe-Garnet and can be either allochromatic or idiochromatic, according to variety. Thus Demantoid is coloured green by chromium impurities but Topazolite is yellow due to iron, which is part of the chemical formula.

Garnet classification: exact classification is one of the most controversial topics in modern Garnet research and a constant source of dispute between mineralogists, gemmologists and the trade. The problem is the above-mentioned isomorphous replacement because of which the end-members (or species) are miscible with each other in endlessly varying proportions.

At least sciencists agree on the chemical composition of Garnet species. Present-day mineralogy has no problem to analyze the chemical composition of Garnets – for instance by means of X-ray diffraction analysis - and thus determine the species. Unfortunately study of the end-members is hampered by the fact that they virtually do not exist in nature. However, this problem was solved, at least in parts, when in the 1950ies scientists succeeded in producing chemically pure synthetic end-members.

Which leaves the problem of miscibility. For Garnets of the Almandine-Pyrope series the name Rhodolite has taken hold. The question, where the line is to be drawn and according to which criteria, is still open, though.

The obvious attribute, colour, unfortunately is no such criterion, for instance because in the 1970ies the so-called Malaya Garnets were found in East Africa. Due to their orange to brownish orange colour they would have been accounted for as Spessartites, when in fact they were of the Spessartite-Pyrope series, which was unknown until then. Furthermore some Malayas contained so much Almandine and so little Spessartite, they were established to belong to the Almandine-Pyrope series, like the Rhodolite or Rose-Garnet.
So, should Malaya Garnets with a high Almandine content be called Rhodolites? Or should Malaya Garnets of the same source and similar colour, but different chemical composition, be split in two groups? Common sense and the gemtrade violently oppose to this.
Another name for Malaya Garnet, used by some gemmologists and gem traders, is Umbalite, after the first source of Malaya Garnet, the Umba Valley in Tanzania. Obviously this also is problematic because orange Almandine-Pyrope and Spessartite-Pyrope Garnets have been found in other corners of the planet, too.

You see, Garnet classification is not an easy thing and the issue will probably not be resolved for some time to come, the more so, because new discoveries keep adding new challenges at irregular intervals.

For practical purposes the following classification will do:

 
refractive index
spec. gravity
colour
Pyrope
1.73 – 1.75
3.65 – 3.80
red with brownish tinge
Almandine
1.78 – 1.82
3.95 – 4.30
pure dark red
Rhodolite(Pyrope-Almandine)
1.75 – 1.78
3.80 – 3.95
red with bluish tinge
Spessartite
1.79 – 1.81
overlaps w. Almandine
4.12 – 4.20

mostly orange, may overlap with Almandine

Grossular
1.73 – 1.76
overlaps w. Pyrope
3.40 – 3.78

yellow to brown, green, overlaps w. Spessartite and Andradite

Andradite
1.880 – 1.888
3.77 – 3.88
green: Demantoid, yellow: Topazolite
overlaps w.  Grossular and Spessartite

Identification: by measurement of physical properties like refractive index and specific gravity. In some cases diagnostic absoprtion spectra (e.g. Almandine) or inclusions, e.g. the famous horse-tail inclusions of Demantoid, can be found. However, nowadays it is no longer possible to identify Demantoid Garnet solely by it´s inclusions because Demantoids have also been discovered in Namibia, Iran and Madagascar, none of which sport horse-tail inclusions.
Sometimes colour provides a clue (a clue, not evidence!).

In the few cases, where these relatively easy to obtain data are not enough for positive identification, professional high-tech mineralogy is called for.

Literatur: Garnet by John D. Rouse, Butterworths 1986, ISBN 0-408-01534-9

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