If you just bought a mortar to honor Mexican food, you should know that before using it, it needs to be prepared to soften it and to prevent small stones from coming off.
Place the rice in the mortar and begin grinding it with the pestle.
2
Make circular movements and try to reach all the walls so that they soften as well as the base.
3
Continue grinding the rice until you feel that the mortar and the base of the pestle have softened a bit and you notice that no more residue comes off.