Metate y Mano
pronounced [mah -TAH-tay - EE MAH-no] (matate).
Also referred to as a "piedra de Moler", this tool is related in lineage to the molcajete. It is used to grind corn and for mashing ingredients to make salsas and purees, and chocolate.
According to Deann and Rick Bayless in their book Authentic Mexican : Regional Cooking, the metate was "The last of the techniques that set Mexican cooking apart is the grinding. It began millennia ago with corn crushed on a rock slab (metate) to make the dough for tortillas. The slab proved good for grinding rehydrated. chills, for nuts and seeds, for cacao beans and even the tender curds of fresh cheese".

Mayan-style metate y mano -
Picture by GourmetSleuth.com
Aztec

Aztec-style Picture by GourmetSleuth.com
The traditional Aztec metate was a
slanted slab with three short legs, similar to the one pictured below. This is
the style of metate frequently found in Mexico. Frequently the surface is more
slanted than the one shown here. The slant allows the user to work the
ingredients downward into a bowl at the end of the metate.
Mayan
The Mayan version sits flat on the ground with a slightly
indented top grinding surface (like the one shown at the top of this
page).
Costa Rican
Metate

Costa Rican Metate Photographer: Unknown
This metate is similar to the Aztec
version with three legs but quite a bit taller. The user would probably sit on a
stool, rather than on the ground when grinding with this tool.
How To Season or Prepare Your Metate Before Use
It is necessary to season the metate prior to use to avoid grit in your food.
1. Submerse the metate and the mano in a basin of water and scrub with a stiff brush. Allow to air dry.
2. Place a handful of uncooked rice on the metate. Use the mano and grind the rice into the surface of the metate. Discard the pulverized rice. Repeat the process until the pulverized rice is white, rather than gray or ash colored. This process will dislodge any bits of loose sand or stone.
3. If you are using your metate for spice grinding follow this procedure. If you are using your metate to grind corn then you should grind one batch of corn (prepared for pozole) but discard the batch.
Place a mixture of 4 cloves of garlic (peeled), 1 teaspoon of cumin (comino) and 1 teaspoon salt, kosher is good, and a teaspoon of pepper on the top of the metate. Using the mano, grind the mixture evenly around the tope surface of the metate. Allow this mixture to remain on the metate overnight. Remove and discard the mixture. Rinse the metate and the mano 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 metate.