The crystallographers report that geodes (as many thundereggs are) can aid breaking addictions. I am inclined to agree from my own experience. The things themselves are so darned addictive that no other addiction stands a chance!

Put simply, thundereggs (or lithophysae) are a structure, not a mineral. They form in rhyolitic larva flows, probably as nodules split on cooling, creating a hollow space inside. This then fills with a agate, quartz, jasper, opal and other things in various combinations. The result is a rock that, when cut in half, reveals a self-contained centre of sometimes magical beauty, surrounded by a rock matrix. The filling is as varied as agate can be, and this variation can be highly location specific. Every bed (site where thundereggs are found) has it’s own characteristics – its own flavour, like some unique variety of wine. Thundereggs from just a few miles away can be almost unrecognizable. That is part of their charm and what makes them so highly collectable. Sit a french Esterel next to an American Friend Ranch egg, next to a Buchanan Ranch egg next to a vivid agate from St Egidien and you might well wonder how these things can be related at all. But the similarities are there in the shapes and structures – the sense of torn rock and the seeming ‘energy’ in their history, which is very different to a sedate classic geode or agate window ornament.

The name Thunderegg comes from american indian legends, but the appearance of these things – the way they seem filled with the frozen energy of torn and stretched rock (far more so than a sedate classic agate) and the layering of colors inside them . . . they really seem to merit their name. Sometimes they look almost organic or as though made of gel or water. Some even seem to have swirling stormclouds frozen inside. Or jagged crystalline lightning or murky rockpools. The Eibonvale Thunderegg Gallery seeks to classify them according to location as far as possible, though true completeness is utterly impossible.