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We found this rock in our backyard. Could someone please tell me what it could be?

Picture of rock

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Please read this guide and add some more detail to the text of the question. – naught101 Sep 8 '14 at 6:14
up vote 16 down vote accepted

This is a chert nodule, which is often found within chalk (the white stuff). The darker stuff (the actual chert) is silica rich and the white stuff (the chalk) is carbon rich.

Chert is often called 'Flint' if it is found within chalk, and depending on your location this may sound more familiar to you.

As for the formation of this rock (which may or may not interest you), the chalk beds are made up of the (exo)skeletons of tiny organisms called Coccoliths. These build up over time to form a huge layer of calcareous deposit which under lithification, forms chalk. During this process, the bits of silica within this deposit will form nodules together, these are the chert nodules.

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Is there anything I can use it for ? Or maybe somene else would find it useful ? – user49126 Jun 23 '14 at 11:27
Well ancient man used to use it to make flint tools, like arrowheads and spears. This is because if you try cracking it, it forms fragments with very sharp edges. So you could feasibly make a simple flint knife, and if you have the know how mount it in a wooden handle. I'm sure there are tutorials online on how to make flint knives. Apart from that there's not really much else to say about it. – AlexLipp Jun 23 '14 at 11:52
The flint will have nucleated around something - often the remains of life. So if you crach them open, there's a chance you might find something like an echinoid, eg. this axe which was deliberately shaped around the enclosed echinoid: – winwaed Jun 23 '14 at 13:54

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