How Do These Creepy Eyeball Rocks Form?

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