ˈhärməˌtōm noun
Etymology: French, from Greek harmos joint + tomē section, from temnein to cut; from its occurrence in crystalline form with an octahedron dividing parallel to the plane that passes through the terminal edges — more at tome
: a mineral (Ba,K)(Al,Si) 2 Si 6 O 16 .6H 2 O consisting of a hydrous silicate of aluminum, barium, and potassium