Where do silverfish blocks spawn in minecraft?

I'm in a survival world with a lot of mountains which makes me nervous because I'm not sure which biome silverfish blocks spawn.


Blocks infested with silverfish only spawn in the extreme hills. They do not spawn naturally in the strongholds, this may be a misconception because silverfish spawned via the spawner may go into a block, which means they can be found there.


Infested Stone (Stone containing silverfish) spawn in extreme mountain biomes.