This \"finger\" of prehnite owes its shape to having formed on top of a crystal of laumontite. It is hollow now because laumontite readily loses water and turns to a powder: the laumontite is no more. But the shape of the prehnite cast allows us to guess laumontite (as opposed to anyhydrite). A very nice Massachusetts specimen, measuring 5.4 x 2.9 x 2.0 cm, without damage.