Rhyolite is an extrusive igneous rock that is often called rainforest jasper because of its rich greens and browns. But it is not a jasper at all.
Rhyolite forms quickly from rapid cooling of magma resulting in crystals so fine they cannot be seen with the naked eyes. Had the minerals cooled slowly underground, they would have formed visible crystals and been called granite.
Rhyolite is typically porphyritic, meaning larger crystals (phenocrysts) grow within pockets formed by gas bubbles that were trapped within the solidifying magma. The phenocrysts are typically quartz and sanidine and may also be amphibole, biotite, and pyroxene.
Image courtesy of gemrockauctions.com.
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