1
Simplified Bragg law

2
Difraction model for opal

A model is only a model. If opal is the unique or rare mineral to diffract light, other structures do it. Morpho butterfly is a good exemple
Important is to understand that the colors of opals are not rainbow colors (like refraction of light in water droplets) but a complex mixing of narrow band colors.
An opal can show an unique color.

3
This ethiopian opal diffract an HeNe laser beam !

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Common opal

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Precious opal

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Where refraction acts to

7
Wing of butterfly Morpho

Here H2O is molecularly linked to S. Nothing to see with free H2O remaining in opal.