Observer

Observers are blocks that emits a redstone signal when an adjacent block is updated.

Breaking
An observer requires a pickaxe to be mined. When mined without a pickaxe, it will drop nothing.

Usage
Observers have two ends. One end observes the block or the space directly facing it, waiting for a block to be placed or broken, or a change in block state there. The other end emits a redstone signal. They are placed similarly to logs. The end that is placed on the block is the side that detects block updates.