Septarian Tumblestone - Large Formed between 50 to 70 million years ago. Septarian nodules are concretions - a volume of sedimentary rock in which a mineral cement fills the spaces between sediment grains. If the cement does not wholly block all the cavities thus formed, water can percolate, often depositing other minerals, particularly Quartz. This gives the nodules complex internal structures, with many different zones and areas of crystallization. This Septarian nodule is showing a binding agent of Calcite, the yellow colour. The darker brown lines are Aragonite and the light brown rock is Limestone. This large tumblestone is from Madagascar and is approx. 4.3 cms x 4.2 cms x 3.2 cms. Check out my other items ! Be sure to add me to your favourites list ! to receive news-letters on my latest items!Please remember if you are making multiple purchases to request an invoice before you pay so I can reduce the postage/shipping costs for you.