Acids (For the sake of this post, acids are any compound that donates a proton, often in the form of a hydrogen atom, when it reacts.) are very reactive, especially the really dangerous ones like HCl and HF, and they can be used to leave marks in things like glass or metal, usually used for decoration. The acid will cut into the glass, which is why you have some glass that had depression marks in them despite the fact that glass isn't phased by things like steel wool.
Source: Chemistry class and personal experimentation.
