Martha Wayne's necklace

A pearl necklace formerly owned by Martha Wayne functioned as an entrance into an elevator that lead to an underground base where Bruce Wayne kept his Batman suit and gadgets.

History
Martha Wayne wore the necklace on the night of her death (it's implied it had some role in what happened). After Martha's and her husband's murder, their son Bruce kept the necklace for years, eventually placing it in his office, inside a square pyramid-shaped glass container that functioned as an entrance into an elevator that lead to the Batcave.