Thunderegg Agates are formed within rhyolite volcanic ash layers. They are rough spherical shapes, varying in size from less than an inch to over a metre long. Thundereggs usually contain centres of chalcedony which may have been fractured followed by deposition of agate, jasper or opal, either uniquely or in combination. A unique characteristic that these specimens have is the fact that they often look like ordinary rocks on the outside, but slicing them in half and polishing them may reveal intricate patterns and colours. These particular specimens are examples of thunderegg agates. Agate is a variety of chalcedony, a cryptocrystalline form of quartz. The agate component contributes to the intriguing internal patterns of the specimens. The specific locality of these specimens is unknown but they can be found in flows of rhyolite lava. They are formed in gas pockets in the lava, which act as moulds. These specimens can be found globally, with specific locations in Germany being particularly abundant. This specimen is part of a larger collection of geological and mineral specimens collected from around Australia (and some parts of the world) and donated to the Burke Museum between 1868-1880. A large percentage of these specimens were collected in Victoria as part of the Geological Survey of Victoria that begun in 1852 (in response to the Gold Rush) to study and map the geology of Victoria. Collecting geological specimens was an important part of mapping and understanding the scientific makeup of the earth. Many of these specimens were sent to research and collecting organisations across Australia, including the Burke Museum, to educate and encourage further study.Two small solid specimens with pale, sandy-coloured exteriors and fractured internal patterns. burke museum, beechworth, geological, geological specimen, thundereggs, thundereggs agate, agate, rhyolite, rhyolite lava, volcanic ash, chalcedony, cryptocrystalline

This specimen was recovered from the Nigretta Falls (previously named the Upper Wannon Falls), Victoria. The falls are fed by the Wannon River that has its head waters in the Grampians mountains. Approx. 10km upstream of the Wannon Falls, Nigretta is a multi-channel segmented cascade of smaller drops and bounces guided by patterns of joints in a much older (Devonian) rhyolitic volcanic rock. Both the Wannon and Nigretta Falls are iconic landscape features, entrenched in the history and identity of the region. These falls are particularly rare as they are the product of volcanic activity, which is unique in the context of the Volcanic Plain of Victoria. Waterfalls of this capacity are generally associated with the Uplands areas of Victoria, rather than the edges of the Volcanic Plain. This specimen is part of a larger collection of geological and mineral specimens collected from around Australia (and some parts of the world) and donated to the Burke Museum between 1868-1880. A large percentage of these specimens were collected in Victoria as part of the Geological Survey of Victoria that begun in 1852 (in response to the Gold Rush) to study and map the geology of Victoria. Collecting geological specimens was an important part of mapping and understanding the scientific makeup of the earth. Many of these specimens were sent to research and collecting organisations across Australia, including the Burke Museum, to educate and encourage further study. A solid, dark grey mineral specimen with pale terracotta coloured inclusions. Rhyodacite is a volcanic rock intermediate in composition between dacite and rhyolite. Rhyodacites form from rapid cooling of lava relatively rich in silica and low in alkali metal oxides.GEOLOGICAL SURVEY / R / S 55 FELSTONE PORPHYburke museum, beechworth, indigo shire, beechworth museum, geological, geological specimen, porphyritic rhyodacite, lava, nigretta falls, upper wannon falls, dacite, rhyolite, volcanic geology