What has science in common with Opal gemstones?

“A lot, just read my story from  a recent visit to Australia”.

1. Introduction

Beside exploring and mining gas, oil, iron, coal, silver, gold and many other commodities, Australia mines Opal gem stones as well. Such Opal mines are rather small, family owned businesses. Already in 1872 the first Opal mine got established in Queensland, while oil and gas exploration started in 1962 only.

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Picture: The following map gives you an overview of what products are mined, where the opal fields are and where the meteorite “Tookoonooka” impacted millions of years ago.

2. The different types of Opal and where they are mined

Australia has three major types of natural sediment hosted precious Opal stones.

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Black Opal, mined mainly near Lightning Ridge, NSW

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White Opal, mined mainly near Cooper Pedy, SA

 

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Boulder Opal: mined mainly near Quilpie, QLD

 

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Picture: The mining centres are widespread and the very first Opals were found in SA in 1849. Today’s main mining towns and the year of discovery are shown on the following map.

3. How Opal got formed by nature’s forces

The precise conditions that allowed Opal to form in nature are still being researched. Therefore, the following information is scientifically not fully correct, since it at least partly verbally transferred over generations.

“Top down theory”: In the “Palaeozoic” times, 540 to 250 million years ago, the whole area known as “Great Artesian Basin” was covered by the “Inland sea”. According to this theory, Silicon (Si) got washed out from the soil and was transported by water into cracks and voids in kaolintic sandstone. Millions of years later, the “Inland sea” retreated and most of the water got evaporated. An amorphous deposit of SiO2*nH2O mineraloid, known as Opal, was left over.

“Bottom up theory”: Some miners believe that hot materials from deep down in the earth was moved towards the surface, where it cooled down and finally developed Opal.

“Tookoonooka meteorite theory”: Another theory is the possibility of different chemical reactions, that started after the impact of the “Tookoonooka meteorite some 120 million years ago and finally lead to the Opal formation.

“Intensive lightning theory” This theory is based on the intensive release of energy from  lightning over millions of years. That, finally lead chemicals to react and form Opal.

“Ongoing formation” Due to the flow of water in the “Great Artesian Basin” the process of Opal formation is believed to be ongoing.

4. The scientific laws that make Opal reflect light

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Picture: From science it is known that the wave length of visible light is in the range of 400nm to 700nm.

Opal molecules are formed in sphere shape and the gap between them is about half the visible light’s wavelength (150nm to 400nm). If the spheres form a regular and smooth pattern, then light beams get diffracted and split up into the spectral light components, following “Brag’s” law.

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Picture: Whether the colour gets reflected at all, depends on the orientation of the surface with regard to the incoming light beam, as well on the gap between the molecules. By nature this gap’s dimension rather reflects blue/green than red colour. Therefore, red colour reflecting Opals are more valuable than blue/green coloured ones. If a stone does not reflect light at all, then it is named “potch” or “common Opal” and is of no value at all.

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