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