How are opals formed?
This is a very simplified answer - Opal is formed by silica deposits, usually developed over time from silica carried in water being left behind in gaps as it passes through the ground.
This usually gives rise to what is termed “potch”, - opal with no colour (a hydrated amorphous form of silica to be more exact).
Why do opals show colours?
Sometimes small pockets of these silica molecules become organised in lattices of flat layers of similar size molecules, this is what becomes coloured opal.
These layers will diffract the light passing through them. As coloured light has different wavelengths the molecule layer will only diffract a certain colour.
Layers of the smaller molecules show violet, increasing sizes of molecules give colours through blue then green, up to the layers of largest molecules showing reds.
These layers are not usually arranged in a completely uniform structure within the opal, hence the different colours and/or patterns showing from different angles as the opal is observed.
To give you an idea of how to demonstrate these layers imagine the following (if you have aphantasia as I do try actually doing it!);
Take a flat sheet of paper and imagine it is a certain colour – when you look at it you have a single flat surface – it would only reflect this one colour when the flat surface is facing towards you.
If you have other sheets of paper of different thicknesses these would be different colours.
If you crumple the paper slightly and then spread it back out you should now have multiple flat surfaces at random angles – this a simplified way of seeing the layers of molecules within the opal as described above.
As you would only see the colour from the individual flat surfaces when they actually face towards you it will hopefully give you an idea of the colours in an opal moving as the stone moves.
How are opals found?
As opal is formed when silica is deposited as described above opals usually have to be mined out of the ground. Sometimes new deposits are found by chance – in Australia most of the main areas of production were initially discovered by people noticing shiny stones on the surface of the ground.
Where are opals found?
The common form of opal is found in many places around the world – even here in the UK! However coloured or gem opal is mainly found in Australia, Ethiopia, Brasil, Mexico, USA, (Idaho and Nevada), Honduras. Other deposits are found in Czech Republic, Slovakia, Hungary, Turkey, Indonesia, Guatemala and Nicaragua. The oldest known opal mines were in Slovakia, thought to have been producing opal since at least Roman times.
No doubt there are as yet undiscovered opal deposits in other places as well!
