Pendants and chains that are plated should not be worn in the shower or tub. Chemicals in soaps will eventually wear off the plating.
Sterling silver, gold, rose gold and platinum pendants and chains will all dull over time. To make them shiny again you can purchase cleaner through the jewelry stores to clean your pieces
Sterling silver can be cleaned nicely at a very low cost using the following:
large square of tinfoil, baking soda, boiling water
Turn large square of tinfoil into a bowl shape (shiny side inside the bowl), place a layer of baking soda in the bottom, put your ring, chain, bracelet, etc on top of the baking soda and pour the boiling water over top. Once the water has cooled remove your piece of jewelry and see how shiny it is again. Do NOT put real opals in sterling silver in this cleaner, use cleaners only for real opals found at the jewelry stores.