Minecraft wiki entry for Development_versions, associated text: Crafted with 2 copper ingots and an amethyst shard.
When used, it zooms in on wherever the player is looking but limits their view to a square.
The scope texture is a glass square with a copper border.
The overlay that limits the view can be removed with a resource pack.
There is a unique animation for using the spyglass, which is visible in a third-person perspective or by another player.