Ribbon, or box joints, whatever you may call them are a popular joint profile choice in stone work. This is the tool for the job here.
Wide square bead jointer
Wide raised square joint, available in 3 sizes
1/2" wide jointer creates 1/4" thick/raised joint
3/4" wide jointer creates 1/4" thick/raised joint
1" wide jointer creates 3/8" thick/raised joint
Solid steel blade with comfortable wooden handle, classic Bon