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 nylium and crimson nylium will transform 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.

Warped nylium can also be obtained through bartering with piglins.

Obtaining
Nylium is mined fastest using a pickaxe, and will only drop itself if mined with a pickaxe.

Bartering
Piglins may barter 1 warped nylium when given a gold ingot.

Sounds
Java Edition:

Trivia

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

Nylium Nylium Nicélio Нилий 菌岩