Pomace refers to pulp—the mashed remains of fruit, seeds, or other plant-stuff, after the juice or oil has been squeezed out. This word is not commonly used but it may appear in recipes for or descriptions of juice, cocktails, oils, baked goods, and some other foods:
After mashing the oranges and removing the juice, put the pomace in a bowl and add sugar.
Pumice is the name for a kind of volcanic stone that is light in weight, low in density, and rough in texture—because it is made of tiny stone bubbles. Pumice is cooled lava—the boiling stone that comes out of volcanoes. These stones are often used for polishing metal or wood, and as ingredients in some cleaning products. One can buy pumice as a powder, or as a piece of rock.
Both of these nouns are names of substances, and so they are both mass nouns, also known as non-count nouns, like water and air. This means that they would both be modified by words like much instead of many, little instead of few, and some instead of a, and they cannot be pluralized. Unless one means “kinds of pomace” (pomaces) or “kinds of pumice” (pumices).