Seasoning a granite mortar and pestle, with intuition for the steps and their order
The rice, salt, peppercorns and garlic aren't hard enough to polish the granite.  There are hardness scales such as the Rockwell hardness scale and Mohs scale for minerals. 

What you are really doing is grinding the granite pestle against the granite mortar. The purpose of having some material in the process is just to get the granite particles to mix into material being ground. The notion is avoid packing the ground off particles onto the surfaces of the mortar and pestle.