Monster Eggs are a dangerous type of block found in Extreme Hills and Extreme Hills Edge Biomes along with Strongholds. They come in three versions and are faster to mine than normal blocks. The four versions that can be found legitimately are Stone Brick, Mossy Stone Brick and Cracked Stone Brick Monster Eggs (in Strongholds) and Stone Monster Eggs (found in Extreme Hills or Extreme Hills Edge biomes). Research is currently underway to see if Dungeons spawned in these two biomes have a chance of containing Cobblestone Monster Eggs, the third type. All of these blocks, when mined, make one Silverfish pop out. If it is attacked, nearby Monster Eggs release their Silverfish. This can easily produce a terrifying swarm.