Thunder eggs are spherical objects which form in some types of silica-rich volcanic rocks (e.g. rhyolites). As the volcanic lava cooled, trapped steam and other gases formed an expanding bubble. Silica and feldspar minerals often crystallise around the bubble or grow crystal fibres which radiate outwards from the its centre. These mineral-filled bubbles with a radiating structure are called spherulites. Internal gas pressure forces the spherulite apart to form a central hollow, later filled with more minerals. Adjacent wedge-shaped segments of the cracked and expanding spherule move outwards and away from each other, helping form the typical star-shaped interior. Silica...