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XL 5.19 in. Opal Thunderegg Nodule - Idaho

XL 5.19 in. Opal Thunderegg Nodule - Idaho

Availability: Out of stock

Precious Opal Thunderegg

Spencer, Idaho, USA

Opal Geode : 3.62 x 3.45 in.
Thunderegg Nodule : 5.19 x 4.70 in.

1 inch = 2.54 cm

Most of these opal geodes are broken down to find the bands of precious opal. Large nodules like this are very rare and almost unheard of to find, after being broken down, most pieces are made into jewelry.

This thunderegg weighs 5 lbs 5.2 oz of precious opal!


Opal from the Spencer Opal Mine is some of the most sought after in the world for it's rarity and color spectrum. A relatively modern mine, in comparison to those from Australia and Ethiopia, the opals present in Idaho span the entire color spectrum and offer their own unique additions to the world of opals in spectacular patterns and color combinations. The formation of these fantastic opals is one that was time consuming and took literally millions of years - beginning 6-4 million years ago, volcanic activity left rhyolite and perlite ash deposits with gas pockets. Before and during our last Ice Age, roughly 1 million to 50,000 years ago, geysers seeped mineral rich solution up and through the ancient volcanic deposit and slowly filled the cavities left by the gas pockets.

A Thundergg is a nodule-like rock, similar to a filled geode, that is formed within rhyolitic volcanic ash layers. 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.