That is because they are soft and the simple act of wiping the dust off of them slowly wears the surface coating away. And woe be to the woman who puts her pearls on before using her hair spray and atomized perfume. These can seriously stain and damage pearls. Opals have been one of the most prized gems throughout history, but they are notoriously delicate. They have a water content and as they dry out they may crack.
Opals are also fragile and can break with the slightest bump.
Not to mention heat sensitive. I knew one poor lady who had a prized opal set in a brooch, where it wasn't likely to be damaged. One night she wore it to a Christmas party. Now, more so than a decade or two ago, using whole crystals in jewelry is quite popular. Natures crystals can be exceptionally beautiful and some people believe they have special properties that are enhanced when left whole.