Lot: Polished Septarian Pebbles - 5 kg (11 lbs)

These are polished septarian "Pebble" or Palm Stone. Each one is about 2-3" wide and fits nicely into the palm of your hand, hence the name. These formed when a lake bed where these were formed dried out. As the mud of the lake bottom dried it contracted and voids formed. The lake bed was subsequently covered with new lake and the older one was buried. The water from the new lake seeped down through the sedimentary rock of the old lake and picked up trace amounts of calcium in passing. when the water entered the voids it would pool and the calcium would deposit out as aragonite and calcite. This is a wholesale lot of 5 kg (11 lbs) of high quality, polished septarian pebbles from Madagascar.

Septarian or septarian nodules are concretions containing angular cavities or cracks, called "septaria", filled with calcite and aragonite. A concretion is a hard, compact mass of rock that often forms around decaying organic matter. In the case of septarian nodules, the concretions formed around decaying sea-life in a marine environment.

The exact mechanism for how the cracks form in the concretions is a mystery. One possible mechanism is the dehydration of the clay-rich core of a concretion, causing it to shrink and crack. The cracks could also be caused by gas expansion produced by the decaying organic matter within a concretion. Earthquakes have also been suggested as yet another mechanism.

The cracks in the concretions are then filled in with minerals such as calcite (yellow), aragonite (brown), and sometimes pyrite, causing the very interesting patterns. They have often been described as looking like dragon's skin. They are frequently found as geodes with hollow, calcite crystal-filled cavities. More rarely, the fossils that originally started the formation of the concretion are still preserved in the septarian.
Pieces 2-3" wide, 5 kg (11 lbs total)