Thursday, 7 March 2013

ANDRADITE PRECIOUS STONES

Green grossular. Below: Andradite garnet
Silicate of calcium and iron, belonging to the garnet group it is named after a Portuguese mineralogist, J. B. D'An drada. 

Crystal system Cubic Appearance

Andradite occurs in crystals in the form of a rhombic dodecahedron, like all garnets, or in convex aggregates of crystals, whose color varies with slight vanetions in the stone's composition, and which therefore have different names: the black variety is called melanite the honey yellow variety topazolite; the green one, demantoid. It can also be blackish brown or blackish red. The crystals may be transparent, semiopaque, or opaque, and often have good luster. 
Known by the general public, it is in demand by collectors and connoisseurs. Simulants and synthetics Green grossular has only been known for a few decades. Green YAG (an artificial product with the structure of garnet, but not containing sill. con) closely resembles it and can be a good imitation. It is not produced synthetically.

Physical properties

These vary slightly according to the composition: the density ranges from 3.81 to 3.86 the refractive index, from 1.882 to 1.889. The hardness is 6.5-7.5, similar to that of other garnets.

Genesis

It is found in metamorphosed limestone and, rarely, in certain intrusive and extrusive igneous rocks. The topazolite and demantoid varieties are found in serpentines often together with asbestos.

Occurrence 

Andradite is mainly found in alluvium in the Soviet Union (Urals) and the United States, but also in Nor-way and Germany.
Demantoid.
This is a pale to clear green variety of andradite garnet graded as being comparable in luster to diamond, hem the name, attributed to it in the latter half of the nineteen century. 

Appearance

The color varies from a cold, very pit green (almost colorless with a green wash), to a mien strong green, not as a rule a very lively color and mine certain shades of green in tourmaline, zircon, or olivine often has good transparency and exceptional luslerldsions in the form of fine, curved fibers of asbestos arehtquent and characteristic. These stones are usually round, mixed, and brilliant cuts: more rarely square on tangular, step cuts. They are normally small. Specimenet more than one carat are uncommon, and those of seem carats are extremely rare. Due to weakening inclusions facets and edges are easily damaged with use. 

Distinctive features

The greenish color combined strong luster, single refraction, and possibly asbestos inclusions, are quite characteristic. However, the physical properties have to be measured to distinguish it korner grossular, green YAG, and occasionally from certain zircons with weak birefringence. 

Value 

Inclusions consisting of thin fibers of amianthus in a demantoid garnet (photomicrograph). Because of its attractive appearance color (particularly when a lively, mid green), and exceptional lure plus its rarity, it is one of the most valuable sera gems.  

Simulants and synthetics

Green YAG, an artificial stance with the structure of garnet, is very similar and has been used to imitate both demantoid and green grossular. For the time being, at any rate, synthetic demantoid produced. 
Writer - Curzio Cipriani and Alessandro Borelli

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