Opal is the hydrated form of amorphous silica. Due to its properties, it has been classified into mineraloid, unlike the crystalline form of silica that has been classified as minerals.
It’s deposited at a relatively very low temperature. Also, it might occur in fissures of any type of rock since it is one of the most commonly available minerals with sandstone, limonite, marl, rhyolite and basalt.
Precious Opal
Precious opal is the opal that exhibits the phenomena called play of the colour which is mainly produced by diffraction of the white light which falls across the micro structure of the orderly array spheres of the silica. Internal structure of the precious opal helps in making it diffract the light on the basis of the conditions wherein it is capable of taking on different colours.
The variant of the natural opal is mainly stated by 2 characteristics of the tone of the body and the transparency.
Body Tone
The basic tone of dark, black and light opal may range from white, colourless, through different shades of black and grey.
Opal in any colour would be opaque, transparent or translucent. When it’s translucent or transparent and clarity of the colour is sharp, it’s known as crystal opal. Opal is also the national gemstone for Australia.
Different Types of Natural Opal
There are different types of natural opal. There’s white opal, black opal, jelly opal and crystal opal. There are certain other types also which are hyalite, hydrophane, fire opal, honey opal, etc.
Black Opal
Black opal is one of the most valuable opal and it comes from lightning ridge. Good quality stone is rarely found. The word “Black opal” doesn’t mean it’s black completely. It just means that it has got a dark tone as compared to white opal. It can be distinguished easily by its blackness of background “colour of the Body” or “tone of the body”.
White Opal
White opal has a basic tone which ranges from the colourless to mid grey and is known as light opal. Some people call it White even though the expression needs to be used only when the cloud of the body is milky. White Opal stone is quite common and due to the tone of its body, usually it doesn’t show the shade and black opal. Nevertheless, white opal could be still magnificent in shade when high quality stone is there.
Boulder Opal
Boulder is the type of a precious opal which has host rocks forming in a natural way as a part of gems. It’s often just a thin vein of the precious opal. It occurs mainly in particular locations. The boulder opal occurs in fillings of the voids or cracks generally in the ironstone boulder. The boulder opal could be light or black on the basis of its appearance when viewed from the surface. The boulder opal has the tendency of cleaving, when cleaved split leaves 2 faces of the opal with natural polished face.
These are some of the types of opal that are most commonly found across the world.
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