The poem pretends to be a deadpan guide to etiquette, describing how different social classes eat stones. The title points to gastroliths, the stones some animals swallow to help them digest food, and the poem turns that idea into a dinner-party custom. The rich treat their stones like diamonds, served in ebony bowls, with rules about taste and even a flag in gems at official events. The newly rich fuss over colors, textures and matching wines, so their stones become décor. Vegetarians and progressives gather "pure" pebbles and swallow them whenever they feel like it, which the speaker describes with a hint of mockery.
Then the tone shifts. The poor "grab whatever they can get": broken cement, beach glass, road gravel. The earlier lines are polished and absurd, but here the language turns plain and bleak. The speaker doesn't need to state a view. The joke about status and fashion suddenly shows that some people are simply hungry. The mood runs from dry satire to quiet, sharp sadness.