Hey Pancho - here is what I've gathered from googling on molcajetes....
"A molcajete is the volcanic rock-vessel used by Mexican cooks (either professional or home) to grind and crush both fresh and dried ingredients used in many Mexican dishes. Both the ancient Aztecs, as well as the Mayans, used molcajetes long before the Spanish explorers and conquerors came to the New World.Before food processors, molcajetes, and their pestle-like counterparts, tejolotes, were instrumental in achieving the perfect consistency of many bases used to flavor various foods. Once broken in, these essentials in Mexican cooking provide the best hard surface for grinding hard spices such as peppercorns and for mashing garlic cloves."
How To Season or Prepare Your Molcajete
It is necessary to season the molcajete prior to using it or you'll get grit in your food.
1. Wash and scrub the interior of the molcajete and the tejolote with water and a stiff brush. Let both objects air dry. Now proceed to steps 2 and 3. We like the combination of both methods or you may select one or the other. This process only needs to be completed one time.
2. Put a handful of uncooked rice in the molcajete. Use the tejolote ( the pestle) and grind the rice into the surface of the molcajete (bowl). Discard the pulverized rice. Repeat the process until the pulverized rice is white, rather than gray or ash colored.
3. Add 4 cloves of garlic (peeled), 1 teaspoon of cumin (comino) and 1 teaspoon salt, kosher is good, and a teaspoon of pepper. Grind the mixture evenly around the interior of the molcajete. Remove and discard the mixture. Rinse the molcajete and tejolote with clear water and allow to dry before storing. Note that these ingredients and quantities can be adjusted to your liking and for the size of your molcajete.
I have been told that anything other than basalt/lava rock will not season properly, and you'll end up with grit in your food. Lots of knock-offs here in souvenir shops - probably hecho en China!