Cog

A Gear was a block that was added to Indev, but removed from the game later. Its data value was later replaced by the Redstone Dust block in Infdev. Gears were first seen at the start of this video, being placed onto the wall of a cliff. A Gear's sprite consists of two parts; the center rod and the animated gear.

Gears still exist in Indev as Redstone had not been implemented then. Gears can only be placed on a side of a block, placing them in a space where two sides are next to each other will cause two Gears to appear on both sides. Gears do not do anything and cannot be obtained without hacking one's inventory. Placed Gears are impossible to destroy; any mining directed at the Gears simply redirects to the block the Gear is on. If the block is destroyed, the Gears do not disappear but instead turn invisible. The gears are still there and will show up again if the block is replaced. Gears placed in mid-air using a map editor are also invisible and will show up if a block is placed next to them.

Trivia

 * Gear data still exists in current versions of Minecraft, so it is possible to be re-added to the game again. Its use will likely have to do with Redstone mechanics.