Prehnite
This species, which is named after its discoverer, Colonel Prehn, is found in nodular, yellow and oil-green stones, of which the latter have very occasionally been cut. It is a little soft, the hardness being only 6 on Mohs’s scale. The double refraction is large in amount, 0·033, biaxial in character, and positive in sign, the least and the greatest of the refractive indices being 1·616 and 1·649 respectively; the specific gravity varies from 2·81 to 2·95. In composition prehnite is a silicate of aluminium and calcium corresponding to the formula H2Ca2Al2(SiO4)3.
The best material has been found at St. Cristophe, Bourg d’Oisans, Dauphiné, France.