How Do These Creepy Eyeball Rocks Form?

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Rocks or mineral crystals can often remind us of other things in our daily lives, but coming across some of THESE rocks might be one of the creepiest experiences a rockhound can have!

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Sources:
https://spookygeology.com/eye-agate-the-rock-that-looks-back-at-you/
https://ieeexplore.ieee.org/abstract/document/6213238
https://gem-a.com/gem-hub/gem-knowledge/optical-chatoyancy-cats-eye-gemstones-sillimanite
https://www.sciencedirect.com/topics/earth-and-planetary-sciences/vesicle
https://www.sciencedirect.com/science/article/abs/pii/S0009281912000670
https://www.mdpi.com/2075-163X/10/11/1037

Images:
https://commons.wikimedia.org/wiki/File:Agate-Quartz-49959.jpg
https://commons.wikimedia.org/wiki/File:Desert_flower.jpg
https://commons.wikimedia.org/wiki/File:Cymophane.jpg
https://commons.wikimedia.org/wiki/File:Eyeballed_by_all_the_eye_agates_(27395607964).jpg
https://commons.wikimedia.org/wiki/File:Agate-_%26_quartz-lined_geode_5_(32375570960).jpg
https://commons.wikimedia.org/wiki/File:Agate_nodule_(%22Lake_Superior_Agate%22)_(Jo_Daviess_County,_Illinois,_USA)_14_(34743267776).jpg
https://www.istockphoto.com/photo/waves-in-light-agate-structure-gm509407138-85755831
https://www.flickr.com/photos/jsjgeology/16128028613