A variety of mushroom tourmaline from the Kat Chay mine, Momeik, near Mogok, Shan state, Myanmar, consists of a black-to-grey single-crystal core from which a single prismatic crystal reaches to the edge of the mushroom, forming a slight protuberance. The rest of the mushroom (∼50% by volume) consists of extremely acicular sub-parallel crystals that diverge toward the edge of the mushroom. The acicular crystals are dominantly colourless to white, with a continuous black zone (2 mm across) near the edge, and pale pink outside the black zone. The composition varies from ∼Na0.75Ca0.05 (Li0.80Al0.70Fe1.10Mn0.30Ti0.10)Al6 Si6 (BO3)3O18(OH)3(OH,F) at the base of the mushroom to ∼Na0.60Ca0.06 (Li1.00Al1.98Fe0.02)Al6 (Si5.35 B0.65)(BO3)3O18(OH)3 (OH,F) close to the edge at the top of the mushroom. The principal substitutions are: (1) YLi + YAl → YFe* + YFe* and (2) TB + YAl → Si + YFe*, but there are five other minor substitutions that are also operative. There are six significant compositional discontinuities at textural boundaries in the mushroom, suggesting that the changes in habit are driven in part by changes in external variables such as T and P, plus possible involvement of new fluid phases.