If you look in a magma block, gravel, quartz or gold ore patch near the top of a forest biome, you can see that the top blocks are nylium. This looks very odd and unnatural. Nether wastes don't have this behavior.
Relates to MC-199005, MC-199006 and MC-199007.
Soul Sand Valleys replace the top blocks with soul sand/soil despite the blocks being overwritten being something else
Gravel patches only generate on the top layer sometimes
Basalt Deltas only overwrite one layer of blocks into basalt, looking slightly unnatural