Minecraft (Pocket Edition)
The above image is of a possible Silverfish Block. They could be anywhere (not really). Waiting. Patiently. Reading newspapers.
Silverfish Blocks are not that common and are primarily found in the Extreme Hills Biome. Typically Silverfish spawn from a Silverfish Block, which looks like a regular stone block. When using tools the block takes longer to mine than it normally would, and when the stone breaks a Silverfish spawns. If the Silverfish is left alone it will cause other Silverfish in the vicinity to Spawn or it will return back to the stone.
The main advice out there seems to be that if a block of Cobblestone, Stone or Stone Bricks is taking an unusual amount of time (more with tools, less by hand) to break then beware. It may be home to Silverfish.
Usage / Purpose
Silverfish are a useless nuisance.
Drops / Yields
Silverfish do not drop anything at this point, and are themselves the drop from a Silverfish Block.
Silverfish are available in both Survival Mode (as a creature spawning from Silverfish Blocks), or in Creative Mode as a Spawn Silverfish Egg.
When in a Stronghold you will encounter a Monster Spawner generating Silverfish near the End Portal.