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THE OPAL

 

  1. COMPOSITION & STRUCTURE

Opals were considered for many years amorphous silicon with a water content in between 2 to 20%.

The use of modern techniques with x rays and electronic microscopes has proved that opals are formed by tiny lepisferas disposed in successive layers of kristobaliitia and tridimita, with same size, packaged in compact structures. Therefore, is not shapeless but semicrystalline. The Opal with high quality is then called Opal C-T ( kritobaliitia-tridimita ).

 

  1. VARIETIES & COLOURS

 

      From the mineralogical point of vie there are two main varieties of Opal: Common and noble. The first has not great range of colours and when is observed against the light has not got beauty. On the other hand, the noble can be transparent, translucent, or even opaque, although some of them present lack of colours variety, they usually show great range in tones that gives their magical beauty calling quickly your attention.

 

This effect called game of colours, should not be mistaken with the opalescence, that happens in the noble opal acting over the parcels of lepisferas like diffraction grilles, the size of the spheres and the light angle of incidence, creates the changing colours effect: from violet to red, green or blue.

 

The colour of the opal (apart  from the game of colours) can be black, grey, white, brown, purple, yellow, orange, green, greenish blue, or colourless. It can be almost transparent, slightly opalescent up to opaque.

Varieties from the gemmological pint of view:

 

  • White Opal: with game of colours, translucent or semi translucent. It can be white, greyish, yellowish, at times turbid and milky.  
  • Black Opal: with game of colours, it can be opaque, dun black or greenish. Is the most appreciated due to beauty and rarity, the good ones reaching high prices in market much higher than the other opals.
  • Opal of water: with a poor game of colours transparent or semi transparent. It can be colourless or yellow pale.
  • Opal of fire: both with or without game of colours. Transparent and semi transparent. It can be yellow,
  • Hydrophane: Translucent, milky and very porous. When is introduced in water becomes more transparent, some of them shows slightly game of colours features. 
  • Matrix opal: Opal that holds part of the mother rock, usually located in the base with the aspect of brown reddish tiny veins. Many experts consider it is not a real variety, just a form of presentation.
Boulder opal (in rock only): with game of colours, too thin to be carved remains in the mother rock. 

 

 

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  • Coloured opals: without game of colours, translucent. Great variety of colours, green, yellow, cherry, bluish, etc..
  • Hyalite: without game of colours, transparent and colourless.

However, there is a wide list with commercial names according to the pattern, shape, and game of colours, many times incorporated in the gemmological designation as if really were part of it. In that list we can highlight.

 

·        Harlequin opal: imitating rhombus effects.

·        Floral opal: effects in leaves shape.

·        Starred opal:  effects of a sky starred.

·        wave opal: imitating the waves effects

  1. PHYSICAL GENERAL PROPERTIES

-         Hardness: Low hardness from 5 to 6.5 in the Mohs scale is very fragile any single knock, or change in temperature can break it. Moreover, water evaporation can cause cracks subtracting value and beauty, happening even to disappear the game of colours.

-         Exfoliation & Fracture: Because of its shapeless, do not show any exfoliation. The fracture is concoidea.

-         Specific gravity:  Very low and variable 2.0/2.25.

 

  1. OPTICAL GENERAL PROPERTIES

-         Brightness: Vitreous, sometimes resinaceous or waxen.  

-         Transparency: As seen before, from very transparent to opaque.

-         Refraction: Is optically isotropic due to its shapeless. The refraction index is low, around 1450.

 

  1. BEHAVIOR TO LOW-RANGE RADIATION.

-         Luminescence under long UV light: Opals presents white fluorescence. Opals with uranium in its composition can show green fluorescence. The opal of fire, sometimes displays fluorescence red or green. Opals of water and blacks are inert.

-         Luminescence under low UV light:  In general, the answers to low range UV light are more intense than to long range  UV light.

 

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