I thought it might be some kind of sharpening stone, but now I have my doubts. it's soft, maybe soapstone soft.
overall:
Attachment 275577
near one end seems to be stamped the word "germany". stamped, not engraved, which suggests that it is synthetic rather than a natural material:
Attachment 275578
the wear pattern seems like it should provide some clues:
Attachment 275579