Prehnite occurs in hydrothermal veins but is most often found as a vein or amygdule mineral in basalt. This mineral was originally discovered in South Africa by Hendrik von Prehn, for whom it was named. Prehnite ranges from colorless to grayish, yellowish green or white and has a vitreous to pearly lustre. A pink variety of Prehnite was discovered at Michigan’s Keweenaw Peninsula.
Chemical composition: Ca2Al2Si3O10(OH)2; Hardness: 6-6.5
Tourmaline is the name given to a large group of minerals sharing the same general properties and crystalline structure but with varying chemistry. Tourmaline occurs in more colors than any other mineral group.
Hardness: 7-7.5
Pearls are the product of fresh-water or sea-water shelled mollusks and can range in color from shades of white to bluish-grey. They are chemically composed of calcium carbonate and have an attractive glossy lustre in their natural state. Pearls have been used as gemstones for over 600 years.
Hardness: 2.5-4
Quartz is a crystalline form of silica. The greatest concentrations of quartz are found in silica-rich igneous and metamorphic rocks. Quartz is one of Earth’s most abundant minerals and is an essential component of granite and many other rocks. Often transparent, Quartz can be found in almost any color. Quartz has piezoelectric properties, meaning it generates an electric charge in response to pressure.
Chemical composition: SiO2; Hardness: 7
Rutile is one of five natural forms of titanium dioxide and the most abundant. It sometimes forms like wheat straw or needles within other crystals such as quartz or topaz.
Chemical composition: TiO2; Hardness: 6-6.5