Wow.  You have some interesting and creative answers to what you thought this stone was.  Thank you for all your answers.

The stone is known as the Coligny Calendar, and was used around 200AD by people who spoke a combination of Latin and Gaulish languages. Most luni-solar calendars contained 355 days. Months with 30 days were marked "lucky" and months with 29 days were "unlucky". Every 2 1/2 years, the astronomers would add a 13th month to make the solar part match the lunar part.

Congrats to Shockz for correctly identifying the stone.

Puffles