Crimson Nylium is the surface block of the crimson forest biome — netherrack with the top face overgrown by a saturated red fungal carpet. Mining it without silk touch returns plain netherrack instead, so the colour version is only obtainable with the right enchantment or by visiting the biome directly. The block accepts plant growth: applying bone meal causes it to spread crimson roots, weeping vines, nether sprouts, and the occasional crimson fungus across adjacent nylium tiles, recreating a miniature forest patch wherever a player settles.

Its most important function is as the only surface that can grow crimson fungi into trees. Place a small crimson fungus on top, apply bone meal, and the fungus erupts into a huge crimson structure roughly twenty blocks tall — a renewable source of stems, shroomlight, weeping vines, and nether wart blocks. The block is also slightly more resistant to fire damage than netherrack, though that distinction matters little since neither catches alight from lava the way Overworld blocks do.