Zeolites have an openness about their structure that allows large ions and molecules to reside and actually move around inside the overall framework. The structure actually contains open channels that allow water and large ions to travel into and out of the crystal structure. The size of these channels controls the size of the molecules or ions and therefore a zeolite like harmotome can act as a chemical sieve, allowing some ions to pass through while blocking others. Color is colorless, white and yellowish. 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.