Like many other mushrooms , L. indigo develop from a nodule , or pinhead , that form within the underground mycelium , a mass of threadlike fungal cells called hyphae that make up the bulk of the organism . Under appropriate environmental conditions of temperature , humidity , and nutrient availability , the visible reproductive structures ( fruit bodies ) are formed . The cap of the fruit body , measuring between 5 and 15 cm ( 2 @.@ 0 and 5 @.@ 9 in ) in diameter , is initially convex and later develop a central depression ; in age it become even more deeply depressed , becoming somewhat funnel @-@ shaped as the edge of the cap lift upward . The margin of the cap is rolled inwards when young , but unrolls and elevates as it mature . The cap surface is indigo blue when fresh , but fades to a paler grayish- or silvery @-@ blue , sometimes with greenish splotches . It is often zonate : marked with concentric lines that form alternating pale and darker zones , and the cap may have dark blue spots , especially towards the edge . Young caps are sticky to the touch .