Formed at the birth of Mt Hay near Rockhampton, Queensland, 120 million years ago, visitors from around the world are amazed by these volcanic gas bubbles.

As molten lava cooled, a rhyolite shell formed around trapped gas bubbles. This shell cracked allowing the gases to escape, leaving behind a hollow cavity. Siliceous liquids containing minerals and impurities entered the cavity and crystallized, sealing the cracks and forming the mysterious centres. These colourful centres are usually made up of agate, chalcedony, jasper, quartz crystal or a combination of these minerals.

These volcanic marvels are very rare, found only in a few countries, and are known by different names. Thunderegg is thought to be a Native American Indian name. They believed the Thundereggs dropped from the sky during thunderstorms. In Europe, Mt Hay Thundereggs are known by various names, such as “Amulet stone”, and “Star Agate”.

In Australia, the Mt Hay stones have some very interesting characteristics that make them unique. When polished on the outside, the partially exposed centre glows if it is held over a light. When viewed under a short-wave ultra-violet light, the centre glows an eerie, emerald green colour, reflecting the enormous forces of a volcano.