What's a thunderegg?

I often hear from people, collectors or not, what it is all about these thundereggs, and what they are. Basicly they are rocks, volcanic rocks. But these rocks, these thundereggs are quite special. They are formed in acid lava flows as spherulithic concretions. After the cooling (and that's still a theory or model) the eggs open in the middle (by gas or by fluids coming in) and get hollow. But not open, some still keep the same. Now some fill up with silicous fluids and the mineralization brings up chalcedony, agate, quartz and many other stuff (calcite, iron minerals, ...). Many thunderegg beds are millions and millions of years old, but the most are from the Cenozoic time period, so rather young stuff.

There are many names for this kind of rock in the world - thunderegg, rhyolite agate, Schneekopfkugel, porphyric agate and many more. But the most interesting one is the term thunderegg. Where does it come from? It is originated in an Indian legend about two mountain gods in Oregon. The legend tells about two mountain spirits living on the tops of Mt. Hood and Mt. Jefferson. They were angry about each other and were fighting all the time. To reach each other they grabbed the eggs of the Thunderbirds living at the mountains and threw them on each other. These eggs were called thundereggs, as the Thunderbird is an important legendary Indian bird living in the mountains of the North American Pacific Northwest.

• chalcedony: the translucent amorphic vareity of quartz (SiO2) which occurs usually in blue, red or greenish tones
• opal: the water-rich variety amorphic variety of quartz which can be shiny (precious) or common (all kinds of colors, usually white, blue, yellow)
• agate: banded chalcedony
• quartz: the crystal form of SiO2, as veins also amorphic
• smokey quartz: the blackish variety of crystalized quartz
• amethyst: the violet toned variety of crystalized quartz
• geode: the technical term for a hollow rock
• moss: structures looking like moss occuring in chalcedony
• plume: more bushy feather-like variety of moss, also occuring in chalcedony
• mudball: a closed thunderegg with no filling
• atoll structure: a basically atoll-look a-like thunderegg interior with a more round middle and a surrounding (closed or open) rim
• conglomerate: many thundereggs (usually smaller ones) gathered in rhyolite, often in layers
• double, tripple, ...: two or more attached thundereggs, often with fused eggs
• egg-in-egg: one egg in another one, looks similar to an atoll structure, but can be different
• ...