The Tower of Pearl was located in one of the Twilit Isles according to the early version of the legendarium in The Book of Lost Tales.
Like the Twilit Isles, it was in the Shadowy Seas, west of Tol Eressëa. There the tower rose pale upon the most western cape of the Twilit Isles. It was built in later days after Tol Eressëa was established, and was much sung in song. During his travels, Eärendel was driven there, where he found the Sleeper in the Tower of Pearl and woke them up with Littleheart's gong.