Thunder egg 10-15mm oval flat beads (ETB01139)

Thunderegg

A thunderegg (or thunder egg) is a nodule-like geological structure, similar to a geode, that is formed within a rhyolitic lava flow.  Thundereggs are rough spheres, most about the size of a baseball—though they can range from less than an inch to over a meter across. They usually contain centres of chalcedony in the form of agate, jasper or opal,[1] either uniquely or in combination. Also frequently encountered are quartz and selenite crystals, as well as various other mineral growths and inclusions. Thundereggs usually look like ordinary rocks on the outside, but slicing them in half and polishing them may reveal intricate patterns and colors.

A characteristic feature of thundereggs is that (like other agates) the individual beds they come from can vary in appearance, though they can maintain a certain specific identity within them.
Thunderegg is not synonymous with either geode or agate. A geode is a simple term for a rock with a hollow in it, often with crystal formation/growth.

A thunderegg on the other hand is a specific geological structure. A thunderegg may be referred to as a geode if it has a hollow in it (see illustration of Gehlberg specimen), but not all geodes are thundereggs because there are many different ways for a hollow to form. Similarly, a thunderegg is just one of the forms that agate can assume.

Thundereggs are found globally wherever conditions are right. In the USA, Oregon especially, remains one of the most famous thunderegg locations. Germany is also an important center for thunderegg agates (especially sites like St Egidien and Gehlberg). Other countries known for their thundereggs include some places in Africa, Poland, Romania, Turkey, Mexico, Argentina, Canada, Australia and France.

Dimensions of beads:

Size: 10.5 x 16.5 x 5  - 15.5 x 22 x 5mm
- this is only an approximate, the size varies slightly.

Length: 16 inches
Finishing: Polished