Thunder Eggs are rough spheres, most around the size of a baseball - though they can range from less than an inch to over a metre across.
They usually contain centres of chalcedony in the form of agate, jasper or opal, either uniquely or in combination. Also frequently encountered are quartz and selinite crystals, as well as various other mineral growths and inclusions.
Thunder eggs usually look like ordinary rocks on the outside, but slicing them in half and polishing them may reveal intricate patterns and colours. Courtesy Wikepedia
Thunderbird Park at Mt Tambourine, Queensland is reputed to be the largest repository of thunder eggs in the world.