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. Bone meal applied to Crimson Nylium sprouts vegetation in a 5×5 area: mostly crimson roots, occasionally a crimson fungus, and roughly once in a hundred uses a warped fungus. Bone meal applied to netherrack placed next to Crimson Nylium, with air above it, converts that netherrack into more Crimson Nylium — and if both nylium types are adjacent, the result is a coin flip.

Its most important function is as the only surface that can grow crimson fungi into huge fungi. Place a small crimson fungus on top, apply bone meal — each use has about a 40% chance of success — and the fungus erupts into a huge crimson structure with a trunk four to twenty-six blocks tall, a renewable source of stems, shroomlight, weeping vines, and nether wart blocks. Crimson Nylium reverts to plain netherrack when an opaque block covers its top, so keep the surface clear when building over it.