Observation Orbs are devices that the Diamonds use to watch activity on their colonies from afar. The one meant for observing the Earth made a cameo appearance in "It Could've Been Great" and reappeared in "Can't Go Back". Another observation orb is shown in the episode "Jungle Moon". Observation orbs are presumably located on the second floor of any given moon base.
Observation Orbs display the activities of the planet(s) they are associated with in real time. An orb can be rotated and touched at certain points to view any location on its particular planet. It is only able to relay visual information; the scenery of the chosen location is projected onto the walls and floor of the room while certain entities at the location are projected as three-dimensional holograms.
The Gems enter a room upstairs in the Moon Base with only a mysterious object in the middle of it. Steven asks what the room is for, and Garnet says that it's not what they came for. Amethyst asks if they can hurry it up because this place gives her the creeps. They continue up the stairs until they reach the top.
It is revealed that Lapis was using the projection orb that was first revealed in "It Could've Been Great" to spy on Steven and the Gems.
In Pearl's story to Sapphire, she tells of how Pink Diamond used the orb to cheerfully observe Amethysts emerge from the Prime Kindergarten, and wish she could be there, leading Pearl to suggest she shapeshift into a Quartz herself to go there physically without upsetting Yellow and Blue Diamond.