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[R.U. Sirius]

A friend gave me this interesting piece of rough, but neither of us could come up with an ID.

Wherever it came from, it's a beautiful example of spherulitic growth - it's all in there: the obvious locations of seeds where crystallization started from, and boundaries between the domains. The material feels a bit dry and fragile, with tendency to break along the fibres that radiate from each of the centres. No idea why and how the orbicular staining happened, it is apparently the result of a separate (later?) geochemical process.

A spherulitic rhyolite of some kind? Any ideas?

[Felicia]

Never saw that one before. Does it react chemically
 in any way? Interesting and attractive specimen.

[Shifter55]

The staining might be the original christobalites from when the rock was fresh(ish) obsidian? These can form quite quickly as I found out last year to my displeasure. From there you could get a slower second stage of growth from the surface of the sperulite as it provides a nucleation site for the acicular crystals.