These are used to make a scorched rice. Here's how I seasoned mine. First, wash and scrub with warm water, but no soap. Stone is porous and will take in the soap flavor, according to some sites. Some sites say to cook salt water in the bowl.
Put the bowl on a low flame to warm it up and evaporate the water. Using vegetable oil, coat the inside of the now-dry pot. Heat it for a while. Remove from heat, cool, and apply oil over every surface. Re-heat on the stove. Eventually, this didn't seem to work, because it left a sticky residue in some places.
Heating in an oven at 350F for a couple hours helped. The pot came out a dark gray, and no signs of stickyness. Also, the "cooked vegetable oil" odor declined. Presumably, the oil re-liquified, spread out, and hardened.
Test by oiling the inside, adding cooked rice, and cooking over a low flame for 15 minutes to see if the rice gets nice and brown.