Opal gemstone

by Manisha G

opal-group-of-gemstonesChemical composition: Hydrous silica di-oxide. Silica with a variable percentage of water (6% to 10%), impurities consisting of the oxides of iron, aluminium, calcium and magnesium are often present.

Characteristics: Non-crystalline, amorphous. A solid silica “gel”.

Varieties: (a) White opal, (b) black opal, both with play of colours (c) Fire opal, red to orange or yellow (d) water opal, transparent with elusive internal play of colour (e) “Potch” opal, milky white with no play of colour.

Physical properties:

Hardness: 5.5 to 6.5; S.G.: 2.1 (Fire opal 2.0).

Refraction: Single, R.I.: 1.44 to 1.46. Play of colour, brilliant rainbow effects which change colour as stone is turned. It is a structural effect.

Other important points: Play of colour is shown only when light is reflected from the stone. In transmitted light, it is usually absent. Some stones gradually lose water at ordinary temperatures and may develop cracks which reduce their value. A number of varieties are recognized in the market.

Harlequin Opal – When colours are in close-set angular patches forming a mosaic.

Flash Opal – shows an individual patch of colour visible only at certain limited angle.

Opal-matrix – Opal which cannot be seperated from the mother rock and which is cut with the rock (matrix).

Black Opal – As the name implies, is almost opaque and has a deep grey to almost black background, often showing a magnificent play of brilliant colour. “Colour” in this variety is usually in flatter layers than in white opal. Hence black opal is generally cut as a low cabochon usually including matrix or common opal “potch” on the back of the stone.

White opal – is usually cuttable as a more rounded cabochon.

Fire opal – is transparent red/orange-yellow with no play of colour.

Opal doublets – are formed of a thin layer of fine opal cemented to a black onyx or black glass base.

Synthetic opal – black and white opal are now being produced commercially.

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