Why do horsetail stems have silica deposits in their outer cell walls?

Horsetail stems in the class Equisetopsida contain deposits of silica and other minerals in their outer cell walls, which provide the plants with a notably abrasive texture.1

This high silica content serves a functional purpose for the plant, acting as a structural reinforcement and a deterrent against herbivores. Historically, the abrasive quality of these stems led to their use as a natural scouring tool for cleaning pots and pans, as well as for polishing various implements.1