Nether Fortress turning into magma
This supporting pillar has turned into magma, but it is holding!
A cave
Note the rough, scattered appearance of various stone types
Frost cavern
A cave that has been frozen
Sand cavern
A sandy cave in a desert biome
Deepslate Stronghold
Strongholds in the Deepslate layer will feature Deepslate bricks, rather than their Stone counterparts