Thundereggs are agates that form in volcanoes. Ancient man, who believed they dropped from the sky during thunderstorms, called them Thundereggs.
Until recently, it was believed they were hurled from a volcano, and cooled and hardened as they flew trough the air. Now, it is believed that eggs form as lava around gas bubbles in the lava flow. Chemicals and gases react as the lava flows to the surface.
The egg is then formed; it cracks under the heat and allows the build up of gas to escape. The hollow is then filled with mineral solutions or silica, which solidifies, consequently forming the different types of centres. The centre of the egg is normally agate, chalcedony or jasper, however you may sometimes find crystalline forms within, such as amethyst, citrine, clear quartz, smoky quartz, calcite or other minerals.
At Mount Hay the lava is of the felsite type, specifically the rhyolite group, consisting mainly of alkaline feldspars and quartz. Mount Hay thundereggs are of two types, spherulites and spheruloids. The former being made up of radiating fibrous crystals of feldspar, the latter made of very fine crystalline quartz and feldspar. Mount Hay thundereggs are over 120 million years old. This is determined by the fossil findings in the surrounding areas.
The thundereggs at Mount Hay are considered to be some of the most attractive in the world because of their spherulitic ryhyolite make-up. Another interesting fact is that there are facilities for the public in Queensland to go ‘fossilling’ for these fascinating and unique eggs. So if ever you’re in Oz, go and visit the Mount Hay Gemstone Tourist Park(near Wycarbah, 41km south west of Rockhampton on the Capricorn Highway) and tell them the Mystic-Mouse web-site sent you!!!