Apophyllite, basic potassium calcium silicate.
Crystal system: Tetragonal.
Colour: Colorless or white, green is much more rare.
Apophyllite is a zeolite, a phyllosilicate with high water content. It occurs as a secondary mineral in basalt cavities in association with other zeolites, commonly Stilbite. Really fine crystalline specimens are found in western India, particularly the Poona district.
The specimen shown is a fine example of bright green Apophyllite in a bed of white Stilbite on a thin basalt matrix. It comes from the Pashan Hills in Poona, India.