You will need to season your mortar and pestle before using. Place two tablespoons of rock salt into the mortar. Grind the salt into a fine powder. Now your mortar and pestle is ready to use. As you use it, the stone will become more seasoned.
Wash with warm water immediately after use to prevent staining.
Never use soap! A stiff wire brush with little water and a little scrubbing and then allow your lava rock molcajete to air dry. Do you see those little porous holes on the molcajete? If you use soap, then soap is what your next batch of guacamole or salsa will taste like.