What is the difference between Common Opal & precious Opal?

What is opal and how does is gets its colour?

Put simply, common opal is opal with no colour. To understand how Opal gets colour please read on...

There much Opal in the earth but very little of it has the 'play of colour'. Read on and discover why the Opal is such a majestic gemstone...

Firstly, and not so majestic...Opal is Hydrated Silica. Broken down means...

• Hydrated - a substance contains water.
• Silica - also known as silicon dioxide, (Silicon is the most abundant mineral in the Earth's crust. Dioxide - 'Di' means to have two, 'oxide' is a chemical compound containing oxygen atoms, therefore Dioxide is two oxygen atons).
  

Therefore, roughly speaking an Opal occurs when netural water passes/is forced through silica rich clay/sandstone collecting silica (the most abundant mineral in the Earth's crust) and pools in empty cavities in the earth/ground until a Gel is formed.

Here is the important part...

If the Gel was at a certain consistency it would allow the particles (Silica Spheres/colloids) to rearranged themselves into an orderly fashion (in-line and of like size), this happened slowly and would take a very very long time. But it was also a race against the water in the Hydrated Silica Gel evaporating. When the water in the gel did evaporate it turned from a gel to a stone solidifying the arranged particles (Silica Spheres/colloids) in a position . If the colloids had the time to get in line with other spheres of the same size then the opal would have the play of colour and if the particles (Silica Spheres/colloids) did not arrange themselves into an orderly fashion then it would be common Opal (also known as potch).

The term "colloid" is specific to the size of the individual particles, which are greater than atoms but small enough not to settle to the bottom of a cavity immediately. The particles may grow to sufficient size to become colloids. It is these colloids that become highly ordered and this is when you get colour (precious opal). Their dynamic behaviour is governed by forces of gravity and sedimentation (tendency for particles in suspension to settle out of the fluid in which they are entrained, and come to rest against a barrier).

So, you can imagine how rare this occurs, so when you see an Opal know that it truly is something that is a true wonder of nature.

How the Hydrated Silica got into the crevices and cavities in the first place is still being debated....

1. The most common belief is that there was a period in time when there was a high concentration of Silica in the atmosphere and this penetrated the Earths crust/entered into the watertable.
   
2. It is due to the great artesian basin - when you look at the area that Opal is found, and then view the are that the Artesian basin covers, it is very simular. The belife is that water was forced up from the basin against gravity (because of pressure in the basin) gathering silica on the way and then pooling in the cavities.
   
3. Water from the atmosphere seeped into the earth collecting silica and then pooled in the cavities.
   

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