M791 - Prehnite is a phyllosilicate of calcium and aluminum. Hardness 6 to 6-1/2. The color varies from light green to yellow, but also colorless, blue and white. Prehnite often forms as stalactitic or botryoidal aggregates, with only just the crest of small crystals showing any faces, which are almost always curved or composite.

Though not a zeolite, it is found associated with minerals such as datolite, calcite, apophyllite, stilbite, laumontite, heulandite etc. i veins and cavities of basaltic rocks, sometimes in granites, syenites, and gneisses.