Infested Block

Infested blocks, or monster eggs, are blocks disguised as stone, cobblestone, or any stone bricks variants, but spawn silverfish when broken. They may not spawn silverfish in creative mode.

Breaking
When the block is broken, whether by a player or by an explosion, a silverfish spawns where it was broken. Infested blocks either break instantly or take longer to break than their normal stone counterparts, and take the same time regardless of the tool:

Identifying
Note blocks placed on infested blocks will produce piano sounds, while those placed on non-infested stone, cobblestone, or stone bricks will produce bass drum sounds.

Monster eggs take longer to break than the stones they appear as, which can be used to tell them apart.

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 are occasionally found in strongholds and igloo basements in place of normal stone bricks. Infested mossy and chiseled stone bricks also spawn in igloos. Infested mossy and cracked stone bricks do not generate naturally within strongholds, with the latter within igloo basements.

Infested stone is most commonly 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. $$, letting a silverfish enter any stone brick variants or cobblestone will turn these blocks into infested stone instead of the corresponding infested block. Because of this, infested stone can also be found outside mountain biomes in this version.

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:

Trivia

 * Despite silverfish being able to infest cobblestone, stone bricks and mossy stone bricks, they cannot infest mossy cobblestone.