Garnet gemstones

by Manisha G

garnet-group-of-gemstonesThis is an isomorphous group of minerals which include several species used in jewellery, the most important of which are grossular (tsavorite), grossularite (hessonite), pyrope, almandine, spessartite and andradite with its sub-variety demantoid.

GENERAL PROPERTIES OF THE GROUP

Chemical Composition : All consists of a double silicate and calcium or magnesium or manganese, aluminium or iron (ferric) or chromium. Hence we get:

 

1 Calcium aluminium silicate – Grossular, hessonite,
         tsavorite
2  Calcium iron silicate – Andradite, Demantoid,
    Topazolite, Melanite
3 Magnesium aluminium silicate – Pyrope
4 Iron Aluminium Silicate – Almandine Garnet.
5 Manganese aluminium silicate – Spessartite
6. Calcium chromium silicate – Uvarovite.

 

These varieties are not sharply separated but owing to isomorphism, grade into one another with the result that garnet of one type usually contains varying amounts of the metals which form garnets of other types. This naturally has an important effect on the physical constants.

The garnets may conveniently be divided into two groups as under:

The Pyralspite group, in which, belong pyrope, almandine and spessartite.

The Ugrandite group comprising of uvarovite, grossular and andradite.

There is usually complete isomorphous replacement between the garnets of each group, but much less between the garnets in different groups.

Crystal Characteristics:

Cubic system, common forms being the rhombic dodecahedron (12 lozenge shaped sides) and the trapezohedron (24 Trapeze shaped sides). Crystals are often well developed examples of either of these forms, generally, but not always, modified or “truncated” by the other form.

GROSSULAR

Chemical Composition : Calcium aluminium Silicate. Iron and manganese partly replace calcium, this accounts for the colour.

Hessonite is the orange red to honey yellow variety of grossularite. (Green transparent variety of this garnet is called Tsavorite. The colour varies from emerald green to yellow green.

Massive hydrated grossularite occurs in South Africa and is known as “Transvaal Jade.” Red, yellow and purple varieties are also known.

PhysicalProperties :

Tsavorite :

Hardness 7.25, R.l.-1.739 to 1.744 single, S.G. – 3.57 to 3.65, Green colour is due to the presence of chromium and Vanadium. Typical spectra i.e. lines in red for chromium and a band in the blue region at 4440 for vanadium are seen.

Hessonite :

Hardness 7.25, S.G.-3.65, R.l. – 1.742 to 1.748 single. Internally the stone often has a treacle granular appearance and numerous small diopside crystal specks may be present.

Massive Green Grossularite :

Hardness 7.25, S.G. 3.36 to 3.55, R.I. 1.70 to 1.73. The massive material of pink and red colour have lower S.G. and R.l. than the green (3.55 and 1.70).

PYROPE

Chemical Composition : Magnesium aluminium silicate. Usually also contains calcium, iron, manganese and chromium. The colour is due to the last three elements. It comes in fine deep crimson red; a slight yellow tinge some times spoils the red. It has been used as a substitute for ruby. It is or has been sold under various misleading names e.g. “Cape ruby”, “Arizona ruby”. The name “Bohemian garnet” refers to rose cut pyrope from that region which was much used in a particular type of Victorian jewellery in the ninteenth century.

Physical properties :

Hardness : 7.25, S.G.– 3.70 to 3.90, R.l. – 1.730 to 1.760, single

Occurrence : Often in association with Olivine rocks or in serpentine. Bohemia (occurs in serpentine and in alluvial soils). South Africa – In blue ground with diamond. Arizona, Sri Lanka, India and Madagascar.

Pyrope is considered to be an indicator mineral for diamonds.

ALMANDINE

Chemical Composition – Iron Aluminum Silicate. Also with variable amount of magnesium, calcium and manganese. Medium to deep red generally with a purplish or brown tinge. When cut as a hollow cabochon known as Carbuncle. Some of these show a faint four rayed star effect or more rarely six rays strongly developed stars are rare.

Physical properties :

Hardness : 7.5, S.G. – 3.90 to 4.20 (average – 4.10), R.l. – 1.76 to 1.81 (average – 1.78)

Spectrum shows very characteristic absorption bands at 5050, 5270 and 5760 A0.

Occurrences : Sri Lanka – in gem gravels. India – from gravels resulting from varying down of gneiss. Plenty available in Orissa, chattisgarh, M.P., A.P., Karnataka, Tamil Nadu. Australia – Adeaide, Rhodesia, Tanzania.

Rhodoite an attractive violet garnet falling between pyrope and amandine in composition and constants. Occurs in Sri Lanka, USA, Tanzania, Rhodesia and India.

Pyrope and almandine are not sharply defined varieties and it is to be noted that the absolutely pure forms are very rare. Between the extremes of the ranges there occurs a complete series of combinations of the two varieties, giving rise to the long and continuous range of R.I. and S.G. values.

SPESSARTITE

Chemical Composition : Manganese Aluminium Silicate, often with some isomorphous replacement by almandine. It has colour ranging from reddish orange to yellow.

Physical Properties :

Hardness : 7.25, S.G.– 4.12 to 4.20, R.l. -1.79 to 1.81 single. Occurrences : USA, Sri Lanka, Brazil.

Properties are near to those of almandine but the colours are different, being more like those of hessonite. Its spectrum shows mangenese bands in the blue and violet region. An almandine bands may be seen if traces of that variety are present.

Malaya garnet is the name given to a orange coloured garnet found in East Africa. The chemical composition is very complex and there it Isomorphism between almandine, pyrope, spessartite.

ANDRADITE (DEMANTOID)

Chemical Composition : Calcium Iron Silicate.

Demantoid : Bright green variety. Yellow, brownish and greenish brown stones also occur.

Andradite in opaque black colour is called Melanite.

Physical Properties:

Hardness : 6 – 6.5, S.G.– 3.82 to 3.85 Lustre – sub adamantine.

Occurrences : Ural mountains – Bobrovka River as rolled pebbles. Parent rock is serpentine. Italy, Zaire, Switzerland and Arizona.

UVAROVITE

Chemical Composition : Chromium Calcium Silicate.

Another green garnet which completes the list but which is not found in the cuttable sizes and transparent enough for gem use. Usually crystals are very small in size.

Hardness : 7.5, S.G. : 3.77, R.I. : 1.87

Detection of Garnets :

The identification of garnet is not often difficult. They are all singly refractive stones. The absorption spectrums are particularly useful for red garnets since most of them contain some percentage of almandine molecules, this is always recognizable in the spectrum once it has been seen. Spessartite has a distinctive spectrum of its own and its properties are sufficiently distinct to make its identification quite certain.

Demantoid with its RI beyond the range of the normal refractometer is unlike other green gems in the same range of values. Green zircon can be identified by its typical spectrum. Whereas sphene cao be identified by high birefringence and green chrome diopside lacks the demantoid inclusions and has different properties.

 

Pyrope/almandine series garnets having an R.l. range of 1.77 can be confused with Siam Rubies. The spectroscope will distinguish between them easily. Red garnet topped doublets can be deceptive, since their colour and R.l. is garnet like. Careful examinations with 10x lens, microscope or immersion methods will detect them.

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