This is the same baking stone that comes with the Fourneau Grande.
The cordierite baking stone is molded with two channels on the top face. The outer channel provides secure indexing for the Grande cloche. The inner channel provides a place to add water to the baking chamber when starting the first phase of baking. That water vaporizes to create a steamy baking environment for your bread, resulting in glossy crusts.
Flip this baking stone over and you will find a flat baking surface, perfect for pizzas and other large baking projects.
If you are looking for a replacement for your Fourneau Grande baking stone, or if you ordered the Kickstarter Cloche kit and wish to add a base stone, this is the right product for you.