Minecraft wiki entry for Combat, associated text: Silverfish are small mobs, which can be spawned from a silverfish spawner, which only naturally generates in strongholds or by breaking an infested block. Infested blocks can be disguised as cobblestone, stone, stone bricks, or variants of stone bricks, and spawn in strongholds, igloo basement, woodland mansion's fake portal room, and at mountains biomes. The breaking speed of infested blocks is the same with any type of tool or bare hand, making it easy to find which blocks are infested blocks. When using the player's hand to break blocks, infested blocks break a lot faster than cobblestone, stone, or stone bricks, but when using a better quality pickaxe, such as iron or diamond, infested blocks break a lot slower than cobblestone, stone, or stone bricks. Infested blocks spawn one silverfish when mined.