 The yellowish to buff cap surface is fibrous and roughened , and reaches up to 12 cm ( 4 @.@ 7 in ) in diameter . The thick stem , roughly the same color as the cap or lighter , is hollowed out into chambers . All parts of the mushroom turn an intense blue color within a few moments of bruising or cutting . The mushroom is edible , despite its hard stem . A less common variety occurs where the color change is to deep violet rather than blue . The bluing reaction results from the oxidation of a chemical called <unk> . 
