Stones, referred to as rocks by Earth scientists, are formed by three different methods. Sedimentary rocks are formed by clasts, or particles that settle and are then cemented together by another agent. Limestone, sandstone, travertine, and onyx are examples of sedimentary rock. Metamorphic stones are those that have undergone a change in composition resulting from intense heat and pressure, typically, a crystallization or recrystallization of the original rock fabric. Marble, slate, serpentine, quartzite, gneiss, and soapstone are examples of metamorphic rocks or stones. Igneous stones are formed by solidification, by cooling of molten or semimolten material. Igneous materials can be intrusive, meaning they cooled below the Earth's surface, or extrusive, meaning they cooled above ground. Granite and basalt are examples of igneous rocks or stones.

Of equal importance to the method of formation is the primary chemical makeup of the stone they are either calcareous or siliceous. Calcareous rocks, such as marble, travertine, and limestone, are made up primarily of calcium carbonate mineralogy. Because calcium carbonate, or the calcite crystal, can be attacked by relatively mild acids, such stones are predictably vulnerable in acidic environments.