To understand how Opal is formed first we have to look at how a rock is made. The sediment is under such high pressure that it forms a solid mass and becomes, quite literally, a ‘rock’. Silica, which is carried by water, gets left behind in any spaces or cavities, the water evaporates, and we are left with rock and silica, which equals opal. It isn’t always rock that becomes opalised. Wood and bone can undergo a similar process, and as the organic matter decomposes, the opal takes its place.
There are some very beautiful specimens of shells and fossils which have become opal.
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