Heulandite shares its structure with the closely related mineral clinoptilolite . Clinoptilolite in fact is considered by some experts to be a high potassium, high silica variety of heulandite. But for now it is considered a distinct species. Natural heulandite specimens can have significant amounts of strontium, potassium, magnesium and barium in their chemical makeup. At times the formula is written to show all these ions and varieties that are enriched in these ions are referred to as strontium rich heulandite for example.
Color is colorless, white, gray, green, pink, yellow, red, brown and black.
Luster is vitreous to pearly on the most prominent pinacoid face and on cleavage surfaces. Crystals are transparent but most commonly translucent. Crystals are often modified by secondary faces with pairs of triangular faces very common. Prismatic and acicular forms are also known and are difficult to identify as heulandite. In aggregate specimens this face can be oriented upward producing a crust of curved pearly faces or it can be oriented to the side where the tops of the crystals jut out like the roof tops of a suburban community.