There is a lot of confusion and misinformation surrounding the various styles of molcajetes available in the U.S. and Mexico. This page should help clear up the confusion.
The first area of confusion is the type of stone used to carve molcajetes. Most of the molcajetes seen in the U.S. are some form of basalt. Depending on the region where the stone is mined the basalt can be very hard or it can be softer and more sandy. The harder the stone, the better the molcajete.
We have seen some concrete molcajetes in the U.S. typically in Mexican grocery stores and in some of the more touristy shops in Tijuana. This is not to imply that all Mexican grocery stores sell concrete molcajetes. We have seen some with "speckled" painted on "texture" and others where you can see rough "mold" marks around the rim and legs.
Smooth VS Rough All basalt stone used for molcajetes is "rough" but the carvers may smooth it out with machine tools.
Molcajetes in the U.S. are typicallly either "plain" or with a pig head. Occasionally you'll see other styles but rarely found in the U.S. We have seen ram head molcajetes, bulls, and even a Turkey. The carving can be quite elaborate.
These are the favorites of Rick Bayless. They are carved using machine tools so they tend to look a little less traditional and more contemporary. The stone is pretty hard and the interior of the bowls usually is left "rough" so you still have a good grinding surface. This style molcajete has become pretty common in the U.S. as well as Mexico.
The typical rough carved molcajete is still hand carved without machine tools. The stone is just a little harder than smoothed stone pieces. This stone can vary in color from dark grey to a lighter tan color. This is probably the most commonly found molcajete in Mexico.
See the additional images below for more molcajete variations.