Silverfish are small, bug-like mobs which have silver skin and black eyes that hide in special blocks found in strongholds. These blocks will look identical to stone bricks, cobblestone, or regular stone but take slightly more time to mine than normal with a pickaxe, and will take noticeably less time to mine without one. When the block is mined, the silverfish pops out and attacks. Destroying the blocks via Creepers or TNT simply kills the silverfish. In regular gameplay, Silverfish blocks occur rarely. If left alone, silverfish can re-enter some blocks, namely cobblestone, regular stone, and stone bricks.
As of 1.0.0 (and possibly earlier), Silverfish can cause damage to a player. The coding behind them allows this to occur when a silverfish is touching a player and a change of the Y-coordinate of either the player or silverfish occurs. This can easily kill a player, since - in a confined space - receiving damage causes the player to perform a short "hop," which counts as a Y-coord change, which allows for the silverfish to damage the player again as the player is coming back down from the initial damage hop. Formerly, Silverfish could not damage the player directly, but would scurry towards the player and push them around, with the tendency to be aggravating and potentially dangerous.
When attacked (but not killed), Silverfish may awaken other Silverfish blocks within a 21x11x21 block area around them. This can lead to a terrifying swarm if there are enough in the vicinity, and an uncontrolled swarm can quickly destroy dozens of stone bricks within a Stronghold. They scurry away after the player moves a short distance.
In addition to the special blocks, monster spawners that spawn silverfish can also be found in strongholds.
Silverfish spawn only from silverfish blocks and monster spawners. In the latter case, they can only spawn in light level 11 or lower, except on top of stone blocks, where they can spawn at any light level. They also cannot spawn within a 5 block distance of any player. These restrictions only apply when spawning from a monster spawner, and not from a Silverfish block.
When attacked, Silverfish may awaken nearby Silverfish blocks. This can lead to a terrifying swarm if there are enough in the vicinity, and in which an uncontrolled swarm can quickly destroy dozens of stone bricks within a Stronghold. They scurry away after the player moves a short distance. They do not have the same sounds as spiders since the 1.0.0 update.
Because of this mob's ability to generate dozens quickly, it's best to try and kill them with arrows, or have your best armor and melee weapon with you.
-The I.D. for the Silverfish block is 46, and can be hacked in through mods or by using the /give command in multiplayer.