A molcajete is a Mexican stone mortar and pestle, used for mashing—and often serving—guacamole, salsas, and other sauces. The older your molcajete the better, Felila says on Chowhound, since an older molcajete is a smooth and seasoned one. But if you happen to have a new one, do you have to worry that grinding a stone pestle (called a tejolote) against a stone mortar will leave grit in your guacamole? Should you use a wooden pestle instead?
You can treat that new molcajete by pounding a handful of uncooked rice and discarding the results, including, obviously, any grit. Spooneb uses rock salt (unlike raw rice, it tends to stay in the molcajete rather than flying all over the place). And talk about seasoning: nokitchen prepped a clean new basalt (lava rock) molcajete by pounding "serranos, white onions and avocado," i.e., the makings of guacamole. In other words, nokitchen thinks grit is really not an issue.