The vulnerability of the various gemstones to acids differs widely. Some must be treated with the utmost care, but others are totally insoluble, for all practical purposes, even in hot concentrated acids.
Undoubtedly, the most susceptible of all gem materials is the pearl. Pearls are composed primarily of calcium carbonate in the form of the mineral aragonite. The carbonates are very soluble even in weak, diluted acids. Thus pearl, coral, shell and malachite must be protected against acids of any kind. These materials, plus turquoise, are among the few that are not exceedingly insoluble. The reason that most gem minerals exist for such great lengths of time in the earth's crust before being discovered is their resistance to being taken into solution by circulating ground waters, many of which are either weakly acidic or weakly basic. Thus they are resistant to both chemical changes and physical forces.
There are other colored stones on which the use of certain acids may be dangerous. Peridot is attached by hydrochloric acid, but too slowly at normal temperatures to be a serious problem in repair work; the same is true of hematite. Turquoise dissolves in the same acid at a rate that demands that this porous gem mineral be kept away from acids. Lapis-lazuli must also be protected from hydrochloric acid, for it decomposes slowly.
Glass, quartz, chalcedony and opal are attacked in varying degrees by the glass-etching acid, hydrofluoric. It dissolves quartz more slowly than glass or opal, but at a significant rate.
Some plastics are attacked by acids, whereas others are highly resistant. Other solvents, such as acetone and carbon tetrachloride, dissolve or soften some plastics.
