What is this?

[-Jones-]

This was given to me. I was told it's 100 million years old. Anyone know what this is? Is it a common fossil?

 

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[PBK]

Cross section of a cephalopod. Very nice, but not scarce.

 

[fossis]

The locals in Africa make a living gathering, & polishing
them, (excavated from stone).

Fossis.................