Septarian Nodule or Concretion


two polished septarian spheres


Septarian from the Latin Septum

A septarian nodule is a distinctive rock whose name comes from the Latin word septum meaning a dividing partition between two tissues or cavities.  Today the word septum is used to describe the fleshy partition that separates one nostril from the other.  This distinctive stone which can easily be identified on sight alone takes on a high polish and should correctly be described as a rock not a mineral because it's made up primarily of three different minerals.  Septarian nodules are sometimes likened to a prehistoric mud ball because of the way they are believed to have formed.

Septarian nodules are composed of the mineral calcite (yellow centre), aragonite or siderite (the brown lines) and limestone which is the outer rock.



Nodule or Concretion?

Although often known simply as septarian, it would be more accurate to refer to this stone as a septarian nodule or concretion.  Both structures are closely related and the terms tend to be used interchangeably.  In geology a nodule refers to a small irregularly shaped knot, mass or lump of crystals or particles with a contrasting composition which means they're not all the same and may even be separated from the formation in which they occurred. A concretion is a body of rock enclosed within sediments that are generally softer and of the same composition as the formation in which they originated.  Concretion also comes from Latin and means 'to grow together' or 'harden' and these structures are often looked upon as being geological curiosities because of the many unusual shapes, sizes and compositions in which they occur.

Septarian nodules are thought to have been formed in shallow lakes as the tide caused the accumulated mass to gently roll back and forth and in doing so, it built up layers of sticky mud which would then have dried out during the hotter months when the water receded. The mud balls were then buried under sediment and the cracks as and when they occurred, were filled through seepage with a crystalline substance such as silica or calcite from shells of dead marine creatures. Crystals subsequently formed which can be seen as the bright yellow centres of septarian.  A thin wall of calcite was also transformed into aragonite (crystal form of calcium carbonate) or siderite which separated the heavy clay exteriors from the crystallized centre.



septarian nodule in a museum display cabinet


London's Natural History Museum | Photo Stone Mania ©




Formation of Septarian Stone

Septarian is known to have formed during the Cretaceous Period which began approximately on hundred and forty five million years ago and ended sixty six million years ago which is around the time the dinosaurs disappeared from Earth.  The stone formed in water as minerals and organic matter accumulated around a centre mass and gradually over millions of years and with the help of various natural processes, hardened and became cemented together. The minerals are likely to have included sandstone which is compacted grains of sand, shale which is compacted mud, siltstone (fine grained silt, sand, clay or other material carried by water and subsequently deposited as sediment) and limestone which is primarily calcium carbonate.  Whilst the exterior of the nodules feature a network of ridges that can have the appearance of turtle shells, the interiors contain distinctive angular cavities or cracks known as 'septaria' from the Latin word septum.  It's not known for certain exactly how the cracks and cavities formed but the most popular theories suggest they were either caused by dehydration or shrinkage of clay or organic matter in the centre of the structure, expansion of gases which may have been caused by decaying organic matter or fracturing or shrinkage caused by earthquakes or compaction.

Describing precisely how septarian nodules formed is not easy and there are many different views and interpretations about how it actually evolved.  Numerous questions remain unanswered and it's a topic that continues to puzzle geologists however the general process is considered to be one that's relatively common in sedimentary rocks



round shaped septarian nodule
Photo courtesy of Stan Celestian




Septarian in Crystal Healing

In crystal healing and according to Judy Hall's Crystal Bible, a septarian nodule is said to encourage one to take care of the earth.  It harmonizes emotions and is generally a joyful stone which helps to bring new ideas and the enthusiasm to follow them through. It enhances the ability to communicate within a group and is emotionally nourishing and calming.



Septarian Nodules in Our Collection


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Further Reading

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