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[Tracy]

What is the mineralogical difference between agate and opal, and why does some opal but not others have "flash" characteristics?

Thanks

[Carles Curto]

On Mineralogy, Opal is a concrete species while agate is just a more or less admited (not always clear to define) variety of Quartz.
Opal has its own characteristics: It is an amorphous hydrated silica oxide, while agate is formerly a cryptocrystaline Quartz or, if you prefer, a chalcedony.
In fact mineralogists are not lovers of varieties, but Quartz varieties have an important role on the history and the literature of Mineralogy and Gemology, then the classic names arrived to us as clear descriptive terms. Words as smoky Quartz, amethyst, agate..., quickly create an image on our heads.
Answering to your question: Being similar but different minerals have different physical characteristics and, between there, different answer to the light. For that reason some optical phenomena as “flash” it is only done on Opal.