Lost Precursor City

The Lost Precursor City is a location in the Crystal Hollows, more specifically the Precursor Remnants, where the Sapphire Crystal can be obtained. Six Automaton Parts are required to claim the crystal, each randomly dropped by killing Automaton or obtained by opening chests in the Crystal Hollows. Delivering all six parts to Professor Robot will unlock the door next to him.

Trivia

 * Right-clicking on the blocks in the door to the Sapphire Crystal will cause them to disappear, although the Crystal will not be claimable unless you give all 6 parts to Professor Robot.