Prehnite
This lovely mineral ranges in color from green to yellow and from grey to colorless. It is transparent to translucent; the color in this semi-precious gem is usually present as a pale warm green glow. Though prehnite belongs to a group of minerals called sheet silicates, which are typically soft and easily cleavable, it has a Mohs hardness of 6-6.5. This hardness makes it a good choice for longwearing jewelry, although long-term exposure to direct sunlight or high heat may cause its delicate color to fade. Prehnite occurs within cavities in hydrothermally altered basalt, other low-grade metamorphosed mafic rocks, or in some calcium carbonate rich rocks that have undergone contact metamorphism. Its green color is imparted by iron impurities present in the hydrous calcium aluminum silicate structure.

References
Klein, Cornelis 2002. Mineral Science. John Wiley and Sons, Inc., New York.
Nesse, W. D. 2000. Introduction to Mineralogy. Oxford University Press, New York.