Nylium

Nylium is a block that generates in the Nether. It has two variants: warped nylium and crimson nylium.

Natural generation
Warped nylium generates in the warped forest biome, and crimson nylium generates in the crimson forest biome.

Post-generation
bone meal on netherrack that is placed next to or adjacent to warped or crimson nylium transforms the netherrack into nylium of that type. No block can be above the netherrack during this process. This transformation can also be done by activating a dispenser with bone meal directly into that netherrack block.

From block loot
Nylium can be mined only with a pickaxe. If mined without pickaxe, it drops nothing.

Breaking
Nylium can be mined only with a pickaxe. If mined without pickaxe, it drops nothing.

Behavior
Similar to grass blocks and mycelium, nylium decays when an opaque block is placed above it. However, unlike grass blocks and mycelium, it does not decay underneath water and lava.

Similar to grass, the player can apply bone meal to nylium. This nylium, along with nearby nylium, generate their corresponding type of roots and both type of fungi (though the majority of the fungi match the nylium). In addition, warped nylium generates Nether sprouts.

$$ fungi must be placed on the same type of nylium to be grown into huge fungi using bone meal. $$ fungi can grow on any type of nylium.

Sounds
Java Edition:

Trivia

 * The name is a portmanteau of "Nether" and "Mycelium", the name of the part of a fungus analogous to roots.