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 Bedrock Editions, Infested  in Java Edition, and Silverfish  in Legacy 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 can be used to identify them. Also, a note block placed on monster eggs will produce the piano sound, while one that's placed on stone or stone bricks will produce the clicks and sticks sound.

In java edition, they will break instantly, regardless of the tool that the player uses.

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 mountains biomes, generating below sea level in veins similar to ores. This type of generation can also override other already generated monster eggs. On certain versions of the game, making a silverfish enter any variant of stone bricks and cobblestone will become this type of monster egg.

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.

Block data
In Bedrock Edition, silverfish blocks 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 Monsternest Blok 97 pt-br:Ovo de Monstro Яйцо монстра 怪物蛋