Agate (“Crazy Lace Agate”) from Mexico. (public display, Dakota Dinosaur Museum, Dickinson, North Dakota, USA) “Agate” is a rockhound/collector term for cavities in rocks (usually sedimentary rocks such as limestone or igneous rocks such as basalt) that have been partially or completely filled with irregularly concentric layers of microcrystalline, fibrous quartz (chalcedony – SiO2). Agate is quartz.
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