Dwarves' Earth Treasure:
What are the Differences Between Amygdaloid Agates, Thundereggs and Sedmentary agates?

.The Amygdaloid Agates
The amygdaloid nodules tend to look rounded, almond-shaped or rotten potatos with some pittings.
and they're always found filling in the bubble-cavities of dark colored basalt and andesite lava beds.

Smooth and rounded nodules with some pittings
Botswana Agate from Africa
Basic and typical shape, no rock shells.
Botswana Agate from Africa
Looks like "rotten potatos"
Laguna agate from Mexico
Usually have white to colorful agates inside
Laguna agate from Mexico

An example of Brazilian Agates as Amygdaoloid agates. Note that they're dome-shaped with flat bottoms.

The Thundereggs
The thundereggs are basically "rock bubbles" found only in rhyolitic lava flows including perlite(primary source) and obsidian.
The thundereggs can be identified by their geometric arranged ridges or "bubbly" surfaces.
When cut, they will show rock "shells" with quartz/agate-filled star-like cavities.

Typical thundereggs with geometric arranged ridges Note the brown rhyolite shells surrounding
agate filled "star-like" cavity
Bubbly surface ("Rock bubbles") Always come with rhyolite rock shells

This is a thunderegg agate core which its shells have been weathered away.
Its star-like shape is an obvious clue to the fact it was a thunderegg.

The Sedimentary Agates
The agates are found in the sedimentary rocks like shale, and cherty limestones and they can be misidentified as the thundereggs.
The outline of agates tend to look jagged due to the way the water were eating away at the interiors of the smooth chert nodules.
Often, the agates will fill in the gaps left by expanded fossil cavities giving them weird cauliflower-like shapes.
The chert nodules can come in saucer or ball-like shapes or just random shaped smooth-skinned nodules.

"Saucer-like" gray nodules from shale beds
Dryhead Agates, Montana
Note the jagged outline of agates and cherty shells
Dryhead Agates, Montana
Cauliflower-like nodules, the remains of fossils or evaporate minerals
Puma Agates from Argentina
Random or irregular shaped,
Puma Agates from Argentina
An classic example of an agate surrounded by its chert shells.
Note some circular banding
Tee Pee Canyon Agate, South Dakota
Note lack of rock shells. That's because the fossil,
evaporate mineral aggregates or chert had been
completely dissolved away and replaced by agate.
Kentucky Agate

The name, "Geode" always refer to any HOLLOW nodules usually lined with crystals and they can occur
in ANY nodules/thundereggs in any rock beds (igenous, sedimentary or metamorphite).

An example of a agate-rimmed geode within a thunderegg.
Sugar bowl Mine, New Mexico