Sclerotium (plural sclerotia) is a solid mass that can appear in the substrate. Species known for producing large amounts of sclerotia are Psilocybe Tampanensis and Psilocybe Mexicana.
Sclerotia somewhat looks like walnut and can grow the size of a tennis ball. There are various methods of cultivating sclerotia. When the cultivator chooses to grow mushrooms of said species, sclerotia will form underneath the casing layer. Another option is to grow only the sclerotium. With this method, the substrate is not introduced to fruiting conditions, and no mushroom forming will take place.
Sclerotia of these strains contain both psilocybin and psilocin.