Nestled at the base of Mt Hay, on what was once a rumbling volcano and the thudereggs you admire today are the fruits of its violent eruptions - "Volcanic Birthstones". Ordinary on the outside, these geo treasures are stunning underneath and their layers tell the story of an incredible geo journey. The thundereggs formed in silica-rich volcanic rocks resulting from ancient lava flow 120 million years ago. These occurred when two forms of crystallization took place creating spherulites and spheruloids. As the lava cooled the star shaped cavity often seen in thundereggs (spheruloids) developed. The cooling caused minute cracks to occur, allowing silica-rich solutions to seep into the cavities. This filled them with minerals such as quartz, agate, chalcedony, quartz crystals, amethyst, jasper and opalite. How Can I Experience Mt Hay? Fossicking, guided tour or camping Heading West See the breathtaking sandstone plateau rising above the plains at Blackdown Tableland National Park. Stop at Blackwater, the Coal Capital of Queensland and home to the International Coal Centre. Heading East Admire the picturesque headlands at Yeppoon and the twin peaks of Double Head, dramatic examples of ancient volcanic plugs.