Agate Geode Agate Ck Qld Click to enlarge image
Agate Geode Agate Ck Qld Image: Gregory Millen
© Australian Museum

Shapes of concretions

The odd shapes of concretions arouse curiosity and they can often be mistaken for fossils, bones, meteorites or other unusual objects. Concretions can have regular shapes like blocks, boxes, pipes, flat discs, canon balls, or even resemble parts of a human body such as a foot or rib. It is sometimes hard to believe that they formed by natural processes. Ironstone concretions are common around Sydney, with its outcrops of iron-rich shale and sandstone.

Formation of concretions

The shape of an ironstone concretion often depends on the way a shale or sandstone bed breaks up into regular blocks of various sizes under the action of weathering. This separation takes place along natural planes of weakness in the rock such as horizontal bedding surfaces and vertical joints (cracks). Before and during this process of separation, ground water soaks in and circulates through the porous rock or along these planes of weakness.

To make ironstone concretions, ground water dissolves iron compounds from the inner portion of a block, then deposits them again as insoluble iron oxide in the outer parts, cementing together grains in the original rock to make that zone harder. There is often brown, yellow or red concentric ironstone banding within the block. This process goes on only while the rock is below the ground water level, but when this level drops, drying and oxidation takes place. Finally, concretions may be released from the surrounding softer rock through weathering. Because they are harder and more resistant, they are found as separate objects in the soil or on the ground.

Concretions can also form by building up of successive layers of material around a nucleus (sand grain, pebble, mineral crystal, fossil or human-made object). Ground water with dissolved iron, silicon, calcium or other chemicals will often drop these as iron oxide, calcium carbonate or silica solids when chemical conditions change, adding them a little at a time as a thin layer. Many such layers may build up, having different concentrations of the compounds, and sometimes showing different colours.

Hollow concretions

Some concretions may be hollow. The centre is empty or filled with loose powdery clay or sand, or a detached hard lump resembling a nut. The loose powder shows that iron oxide formerly cementing the grains has been drawn away from the middle and towards the outside, contributing to the hard iron oxide shell. If the centre is empty, cracks have allowed the loose powder to escape.

If a loose 'nut' is present, there has been some internal shrinkage when the concretion dried out. Both outer shell and loose 'nut' may show banding. Sometimes the 'nut' can be heard rattling inside the concretion when it is shaken.

Thunder eggs

Thunder eggs are spherical objects which form in some types of silica-rich volcanic rocks (e.g. rhyolites). As the volcanic lava cooled, trapped steam and other gases formed an expanding bubble. Silica and feldspar minerals often crystallise around the bubble or grow crystal fibres which radiate outwards from the its centre. These mineral-filled bubbles with a radiating structure are called spherulites.

Internal gas pressure forces the spherulite apart to form a central hollow, later filled with more minerals. Adjacent wedge-shaped segments of the cracked and expanding spherule move outwards and away from each other, helping form the typical star-shaped interior. Silica gels and clays filling the cavity can later dry out, shrink and crack, producing more internal structures such as interesting patterns of mineral-filled cracks.

Later, silica-rich solutions may enter the cavity and fill it with banded agate, chalcedony, clear quartz crystals or amethyst. Solutions of different composition seep in at various times, leaving behind several layers of different minerals. Well-known localities for Australian thunder eggs are Mt Hay, Eumundi, Agate Creek and Mt Tamborine, in Queensland, and Boggabri, Barrington Tops and Murwillumbah in New South Wales.


Geodes are hollow, crystal-lined globular rock cavities found in sedimentary rocks like limestone (calcium carbonate) and dolomite (calcium magnesium carbonate) or in volcanic rocks. In sedimentary rocks, geodes may form by dissolving out cavities by ground water and re-depositing of minerals as crystals, which usually point towards the centre of the cavity.

They may also form in pre-existing concretions or in spaces left by expansion of the rock under internal fluid pressure. They often have an outer shell of chalcedony, a crystal lining of quartz, carbonates or other minerals, and can be over a metre in diameter. Sedimentary geodes made of calcite and dolomite have been found in the Muswellbrook-Singleton area, New South Wales.

Volcanic geodes

Some volcanic lavas (e.g. basalts), have round or almond-shaped gas holes (called amygdales), partially or completely filled with calcite, chalcedony, agate (banded chalcedony), crystallised colourless or amethyst quartz or other minerals. Ground waters carrying dissolved silica may seep into the lava while it is cooling, depositing silica minerals in the cavities. The agate and chalcedony were initially in a jelly-like state but hardened as they dried out. Examples have been found at Agate Creek, Monto and Murgon, Queensland; Narrabri, Merriwah, Werris Creek and Bellata, New South Wales and along the lower reaches of the Snowy River, Victoria.


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