Thunderegg Crystal Mine at Thunderbird Park

What are thundereggs and why are they so special?

The mineral rich seam that runs through Mount Tamborine was discovered at Thunderbird Park in the 1930’s and opened to the Public in 1967. Today, the ground at 300m by 50m open - cut mine has an endless supply of beautiful thundereggs.

The name of 'thunderegg' originated in the United States, and refers to spherulite (spherical body generally occurring in glassy rocks, especially silica rich rhyolites), a geological phenomenon greatly prized by collectors and amateur lapidaries.

The formation of thundereggs is much of a mystery, but it is thought that during volcanic upheavals in far off times bubbles formed in silica rich material such as rhyolite (an igneous, volcanic (extrusive) rock, of felsic (silicon rich) composition). As cooling took place hot gases and steam expanded nodules in which they were trapped, and in these cavities some mineral structures joined together. Shrinkage of the material on cooling produced the star shaped cavity which is characteristic of many thundereggs. Silica rich minerals entered the cavity, and solidified into chalcedony, jasper, quartz, agates and other beautiful minerals. Crystals were deposited on the walls,
and the repetition of these processes produced the interesting patterns and colours which give thundereggs their universal appeal.

As the lava cooled crystallization of feldspar fibres occurred in a radical fashion around bubbles, and Cristobalite (a polymorph of quartz, meaning that it is composed of the same chemistry, SiO2, but has a different structure) filled the interstices in the radial network. Steam and gases which accumulated in the nodules interior caused expansion of the spherulite, and in some cases a large central cavity was formed by the internal gas pressure forcing the spherulite apart. Sometimes part
of the whole of the cavity was subsequently filled with banded agate, chalcedony and quartz.

A beautiful display of these silica minerals and a wide variety of interesting structures make spherulites of special interest to gemologists and lapidaries.

The most impressive and largest collection of thundereggs in the world is at Cedar Creek, Tamborine Mountain in Queensland. The thunderegg deposit here is on private property spread over 320 acres of lush rainforest. The thundereggs are found in all sizes, from a few grams in
weight to over 600 kilograms, and they nearly all contain interesting fillings of agate and other varieties of quartz.