I thought it might be some kind of sharpening stone, but now I have my doubts. it's soft, maybe soapstone soft.

overall:
tumblr_mu2hciwWTk1qhrm32o1_500.jpg

near one end seems to be stamped the word "germany". stamped, not engraved, which suggests that it is synthetic rather than a natural material:

tumblr_mu2hccZNtR1qhrm32o1_500.jpg


the wear pattern seems like it should provide some clues:
tumblr_mu2hbxX6GV1qhrm32o1_500.jpg