Gear

The gear was a block that could be placed on walls, but had no clearly defined or implemented functionality. It is believed that gears were intended to be used in mechanisms, similarly to redstone dust.

Obtaining
It could only be obtained by inventory editing and was at first invisible in the inventory. Later it became visible.

Usage
When edited into the game, a placed gear would show nothing in its block space and was invisible unless horizontally adjacent to one or more blocks. The placed gear would then show an animated gear sprite on the near side of each of those blocks. It was not possible to destroy a placed gear by mining a block that its gear sprite was shown on, because the placed gear itself was actually adjacent to that block. If the block a gear sprite was shown on was destroyed, the adjacent placed gear would no longer show that sprite. Removing all blocks adjacent to a placed gear would remove all of its visible gear sprites, leaving the placed gear invisible. Placed gears could not be destroyed directly by the player; attempts to do so would phase through to the block behind it, much like water. Placed gears could only be removed by letting water (and presumably lava) flow into them or by explosions.

Gear's sprite consisted of two parts: The center rod and the animated gear:

ID
The ID is the same as redstone in current versions: number 55.

Gallery
Engrenage Ingranaggio 歯車 Engrenagem