Information on Turquoise stones

by Manisha G

Turquoise:

turquoise-gemstones

Chemical composition:

Hydrous phosphate of aluminium & copper with some iron.

Cryptocrystalline: Triclinic

Varieties:

Persssian: fancy deep blue, the most valuable.

Egyptian: tendency to green, somewhat glossy appearance and more translucent.

American: pale to green.

Physical Properties:

Hardness: 5.5 to 6

S.G.: 2.60 – 2.85. American types are more porousand have lower density.

Lusture: waxy; Diaphenity: depends partly on thickness and most turquoise are semi-translucent in thin pieces. It is not often possible to determine an R.I. that is 1.610 – 1.650.

Other important points:

Valued for its colour. However in some cases, the colour tends to fade on long exposure to light. Some stones of good colour in the moist conditions of the mine, become bleached on being removed to the open air. The original colour may be temporarily restored by placing in damp earth, a trick which has been restored to in the locality of the mines in order to deceive intending purchasers. Immersion in a solution of ammonia will sometime restore the blue colour to a stone, which has become green, but the change is not permanent. Pale turquoise is sometimes stained blue chemically, but if a drop of ammonia is placed on back of the stone, the spot becomes green or colourless and the device is detected. The porous nature of the stone renders the colour liable to the damaging effects of grease and perspiration during wear. Sometimes cut to include veins of brown limonite in which it is found, then known as turquoise matrix. Much modern material is reputed to be waxed or oiled to improve the colour and polish. This is difficult to detect without damaging the stone. The porous American material is sometimes “bonded” with plastic to harden it and make it more compact. Synthetic Turquoise is being produced commercially and can be detected only by sophisticated method.

Cut: Cabochon, as bead or carved.

Treatment: Dyeing, waxing, oiling, plastic impregnation.

Detection: This material is glossy in lusture and often reveals its nature by hemispherical pits in the surface-bubbles which have been cut through in polishing.

Plastic imitations are of low density and poor lusture. Other subterfuges consists of improving the colour with stains. These are usually only on the surface. A compressed precipitate of aluminium phosphate, with suitable colouring matter, bonded in plastics has also been used. So have various types of ceramic materials. The compressed material is of good appearance and has similar hardness and specific gravity. Imitations of turquoise and more particularly the waxing to improve the colour are not easily detected without destruction or damage to the stone.

When the material is sufficiently thin to allow light to pass through the edges, the spectroscope may be used to detect characteristic absorption bands in the violet i.e. 4200, 4320, 4600A.

 

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