Prehnite is orthorhombic, which means its crystallizes in the shape of rectangular box that is equal to two cubes stuck together. It's not usually easily seen as it is often inside a stalactite (long crystal formed by dripping water that you see hanging inside caves) or like a bunch of grapes within another mineral where just a small portion of some of the grapes are on the surface.
Prehnite breaks easily in uneven pieces. It has a translucent, glass-like appearance and its color runs from very pale yellow to light green, but sometimes light blue. It was first discovered, or at least written about, in 1788, in South Africa. The crystal was named for a Dutch commander, Col. Hendrik Von Prehn, in charge of the miliary in the Dutch colony in the Cape of Good Hope.
Prehnite that is gem-quality comes from Wave Hill Station in Northern Territory, Australia.