Prismarine Crystal

Prismarine crystals have most of the same functions that they do in regular Minecraft

Additionally, in Horizons end you can craft sea lanterns using four prismarine crystals, using the custom crafting recipes

Unlike in regular Minecraft, prismarine crystals can be easily obtained by mining certain asteroids

Notably, A special use for prismarine crystals in Horizons end is to be placed in the upper slot of a furnace in multi-block machines. This is most common in multi-block machines that take a single input material.