Care and Feeding of Molcajetes
Molcajetes are used to make guacamole and grind spices and are made from volcanic rock. Since the hardened lava produces a loose structure, these mortars and pestles must be treated before use so that loose pieces of the rock do not get ground into the food.
Place your molcajete in the sink and wet thoroughly. Also the pestle must be rinsed thoroughly. Take handfuls of raw rice and place in the molcajete. Add some cold tap water and begin to grind the rice into the bowl. Repeat several times, making sure that you have ground the entire interior surface. Rotate the pestle as well so that its surface is cleaned as well.
This needs to be done until the water runs clear. No bits of stone should be coming off as you rinse.