http://www.forrestrace.com/white-opal/
Can you please tell me a bit about opal ?
Opal is very beautiful, it’s just so pretty. So my question is: What causes or how can one rock have so many different colors?
If you have any information about opal rocks, share it with me please.
Thank you in advance.
http://www.opalsdownunder.com.au/images/articles/white.jpg
Opal is hydrated SiO2. There are many varieties of opal but from your description, I assume you’re referring to fire opal. Opal forms in hydrothermal veins where silica is precipitating and consolidating water from the environment into it’s crystal structure, as high as 20%. You can find opal in any hotspring deposits, petrified wood, holes in volcanic rocks. Opal can also be formed by replacing siliceous fossils like diatoms.
When silica in opal precipitates, it does so in extremely tiny spheres, all packed together in a very orderly and packed lattice. As light passes through these spheres, it gets bounced around and refracted and difracted so much that light waves begin to interfere with each other. This is what causes the spectacular color in opal. However, opal can only maintain the spherical arrangement because it is so hydrous. If you take and opal and cook all the water out, it becomes a dull, ordinary chert. Opals can become anhydrous, just by being in the sun too long.
Edit- Just a side note after reading tick tocks post… Opal is not a fossil in itself. Rather, opal replaces silica minerals in fossils. I’m not sure if this is the approach that Tick Tock’s book was taking or not. But just to clarify, opal VERY commonly comes in the forms of fossils, especially diatoms and petrified wood. The way this happens is for example, a tree dies and gets deposited in say a lake or stream bed. The tree soaks up water which is high in silicic acid. Before the tree decomposes, it is rapidly buried and over the years it becomes part of a hydrothermal vein. The silica then alters to form opal in the way described above. The same goes for diatoms and forams which are siliceous organisms in lakes and oceans. The diatoms die, there silica skeletons drift to the bottom of the ocean and form chert, which can, under the previously mentioned conditions, become opal.
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