Utah Thundereggs - A Surprise Rock 
 
A thunderegg is an agate that is formed in a particular way that creates a center of agate usually surrouded by a frame of contrasting color. As Wikipedia puts it, "A thunderegg is just one of the forms that agate can assume."
 
The first thing that needs to be present for a thunderegg to form is a hollow in the ground created by a gas bubble in volcanic rhyolite. The geology of the Cascade range in Oregon was just right for creating thundereggs and the whole state is famous as a place where they can be found. But they're also found all around the world because the volcanic conditions that form them are found all around the world too. For example, rhyolite is also the source of many jaspers such as wonderstone.
 
The gas bubble hollow is first filled with chalcedony by water percolating through the rhyolite. With many thundereggs, this initial chalcedony is cracked open and the inner core is replaced by the silica that forms a more pure and often translucent agate inside.
 
The final result is often the familiar patterns that Oregon thundereggs are known for. If you're interested, I have a very long and very technical book, in digital PDF form, that explains it completely.
 
The Utah Thundereggs that are in my dad's collection are different. The outer agate is a sparkling layer (because it's made of relatively large crystals) with a red agate center.
 
I haven't seen these Utah thundereggs anywhere else. You'll have a rare and unique rock if you order this one.