Infested Block

Infested blocks, also known as monster eggs or silverfish blocks, are blocks disguised as stone or stone bricks, but spawn silverfish when broken.

Breaking
Infested blocks take the same amount of time to break as the block they're disguised as or instantly with any tool. When the block is broken, whether by a player or by an explosion, a silverfish spawns where it was broken. Infested blocks will typically take longer to break than their normal stone counterparts, which can be used to identify them. Also, note blocks placed on infested blocks will produce piano sounds, while those placed on stone or stone bricks will produce bass drum sounds.

Mining an infested block with Silk Touch drops its non-infested counterpart, and does not spawn the silverfish.

A silverfish will not spawn if the block is destroyed by the ender dragon.

Natural generation
Infested stone bricks, cracked stone bricks, and mossy stone bricks are occasionally found in strongholds and igloo basements in place of normal stone bricks. Infested chiseled stone bricks also spawn in igloos.

Infested stone is rarely found in mountains biomes, generating below sea level in veins similar to ores. This type of generation can also override other already generated infested blocks. On certain versions of the game (i.e. Bedrock Edition), letting a silverfish enter any stone brick variants or cobblestone will turn these blocks into infested stone instead of retaining the texture like in Java Edition.

Infested cobblestone is found in the rarely generated "fake secret end portal" room of woodland mansions.

Creative mode
Infested blocks are available in the Creative inventory. When an infested block is destroyed by a player in Creative mode, a silverfish is not spawned.

Usage
When mined, infested blocks will spawn a silverfish which will instantly attack the player. If the player attacks a silverfish directly with a sword, bow or potion of Harming, or when a silverfish takes poison damage, nearby infested blocks will break, spawning other aggressive silverfish. If the silverfish is killed in one hit then nearby infested blocks will not break, and no silverfish will spawn from them.

Block data
In Bedrock Edition, monster eggs use the following data values:

Issues
Blok rybenky Silberfischchen-Stein Piedra con lepisma Bloc infesté Blocco infestato シルバーフィッシュ入りのブロック 벌레 먹은 블록 Monsternest Blok 97 Ovo de Monstro Яйцо монстра 怪物蛋