Monster Egg

Silverfish Stone, also known as Monster Eggs, is a block that spawns Silverfish when broken. Silverfish Stone has identical looks to either Stone, Cobblestone or any kind of Stone Brick, and are naturally found in Strongholds and rarely in Extreme Hills biomes, hidden among the real blocks.

Detecting
When breaking it with a Pickaxe it takes noticeably longer to mine than regular Stone, Cobblestone or Stone Bricks, making it relatively easy to distinguish from the regular blocks, if one is careful enough. The block can also be detected as it is destroyed by hand fairly quickly, unlike regular stone-related blocks.

Threat to Players
Silverfish Stone is meant to deceive players. When mined, it will instantly spawn a Silverfish which will instantly attack the player. Also if the player attacks a Silverfish directly, nearby Silverfish Stone will break, spawning other aggressive Silverfish.

Trivia

 * If the block is destroyed by a explosion (Creepers, TNT etc.) the Silverfish will die in that block.
 * When a Silverfish is not pursuing the player, the Silverfish will go to the nearest Stone, Cobblestone or Stone Brick and go inside it, turning the block into Silverfish Stone.
 * When mined, the Silverfish will have full health.
 * Silverfish Stone can be made by placing Stone, Cobblestone or Stone Bricks and tapping a Silverfish Spawn Egg on it, as the silverfish will go inside it, converting the block into a Silverfish Stone.