Agate is a rock made of translucent cryptocrystalline quartz. This form of very fine-grained quartz is common and occurs in a number of different geologic environments. It is often formed as a chemical precipitate in fractures and voids in host volcanic rocks from silicon rich fluids moving through the rock. Changes in colors and patterns are related to variations in fluid flow and minor element variations, such as a change in the amount of iron or magnesium in the precipitating fluid.

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