Epistilbite is one of the rarer zeolites, a popular group of minerals to collect. Epistilbite commonly forms in the petrified bubbles (called vesicles) of volcanic rocks that have had a slight amount of exposure to metamorphism. It also forms in some pegmatites. Epistilbite's name means over stilbite perhaps in allusion to crystals of epistilbite that form over crystals of stilbite, a closely related zeolite. Epistilbite has similar crystal habits to stilbite although it rarely forms the large, impressive clusters that has made stilbite so famous.
Epistilbite's structure has a typical zeolite openness 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 epistilbite can act as a chemical sieve, allowing some ions to pass through while blocking others.