Cut and polished ammonite
Several stages of crystallisation preserved this ammonite shell. Pyrite (fool’s gold) crystallised and replaced the shell and chamber walls, then, calcite crystals grew within the centre of each chamber.
The iron pyrites form because, at times, the sea bed became poisonous and stagnant. Without the presence of oxygen, iron and sulphur developed in the sediment and formed iron pyrites. The calcite formed later from calcium-rich water percolating through the rocks. Any cracks or cavities provide the space for the calcium to crystallize as calcium carbonate, or calcite crystal.
Find out more about ammonites on the molluscs pages.