A pearl is mostly calcium carbonate, which is susceptible to even a weak acid solution. The crystals of calcium carbonate are converted by acetic acid (vinegar) into calcium, which dissolves in the residual water, and carbonate that effervesces as bubbles of carbon dioxide, the formula for which is CaCO3 + CH3COOH --> Ca + H2O + CO2.
This is all cleopatra's fault for drinking that pearl and vinegar concoction (called "magister") and sharing it with Marc Antony. A waste of a good pearl if you ask me...