Infested Block

A monster egg is a block that masquerades as a typical stone-type block, but which spawns silverfish when broken.

They are named  Monster Egg in the Java and Bedrock editions, and Silverfish  in Console Edition, where  is the name of the block they are disguised as.

Breaking
Monster eggs take the same amount of time to break with any tool. When the block is broken, whether by a player or by an explosion, a silverfish spawns where it was broken. Monster eggs will typically take longer to break than their normal stone counterparts, which may be helpful in identifying and avoiding them.

Mining a monster egg with a Silk Touch tool drops the block that the monster egg is disguised as, and does not spawn the silverfish.

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

Natural generation
Stone brick, cracked stone brick, and mossy stone brick monster eggs are occasionally found in Strongholds and igloo basements, in place of normal stone bricks. Chiseled stone bricks monster eggs also spawn in igloos.

Stone monster eggs are rarely found in the Extreme Hills biome, generating below sea level in veins similar to ores.

Cobblestone monster eggs are found in the rarely generated "fake secret end portal" room of woodland mansions.

Creative mode
Monster eggs are available in the Creative inventory in the Java and Bedrock editions.

When a monster egg is destroyed by a player in Creative mode, a silverfish is not spawned.

Usage
When mined, monster eggs 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 monster eggs will break, spawning other aggressive silverfish. If the silverfish is killed in one hit, nearby monster eggs will not break, and no silverfish will spawn from them.

Trivia

 * Majority of the monster eggs in strongholds (generated naturally) are in the form of regular stone bricks. Cracked and moss variants can also be a monster egg but this is rare to none unless a silverfish enters these blocks. Also, exposed stone within an area will become this block if the situation on a last sentence happens.

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