Luster is vitreous. Crystals are transparent to translucent, often milky. Although examples showing a perfectly formed twin crystal are scarce, twinning is wide spread and almost all crystals of harmotome are twinned. Aggregates can be radiating and columnar. Cleavage is poor in one direction.
Fracture is conchoidal to uneven.
Hardness is 4.5. Streak is white. Associated Minerals are quartz , heulandite , stilbite , pyrite , brewsterite , edingtonite , barite and other barium and some manganese minerals. Best Field Indicators are crystal habit, twinning, density, hardness and associations.