The color is due to the presence of oxide of nickel. This stone is found principally in Silesia, but also in Siberia and the United States. Large pieces of chrysoprase are rare, and even the best specimens lose their color in course of time. PRASE. A translucent, spotted leek-green, green quartz, which loses its polish on exposure to the air, is known as prase. This stone is found principally in the iron mines of Brietenbaum, Saxony, and also in Brittany, the Tyrol, Scotland, Salzburg, Finland, and the United States. Prase is sometimes known commercially as “mother of emerald,” and a greenish crystalline quartz is also often called prase. PLASMA. Plasma is a dark grass-green quartz, feebly translucent, and is sometimes covered with white or yellow spots. Plasma is somewhat lighter in weight than the heliotrope and does not take as fine a polish. This stone is found in India, China, and in the Black Forest, Germany. CHALCEDONY. Chalcedony is cloudy or translucent, white, yellowish-gray, blackish-brown, light to dark-blue, milky-white, and black. This quartz is sometimes nearly transparent, waxy in lustre, and in some varieties has a light gray and transparent base with dark cloudy spots. This last variety is called “cloudy chalcedony”. Another kind, with gray and white stripes alternating, is known as chalcedonyx. Iceland, the Faroe Islands, Hüttenberg, Loben, Saxony, Hungary, Nubia, Nova Scotia, Oberstein, Ceylon, India, Siberia, Carinthia, the Hebrides, the United States, and Canada are places where chalcedony is found. AGATES. Agate is an improved variety of chalcedony and comprises the following kinds. Banded or ribbon agate, running in delicate parallel layers. Eye agate, forming concentric rings with a dark centre, giving the appearance of a human eye. Fortification agate, running in circular parallel zigzag lines like the walls of a fortress. Rainbow agate is a thin or concentric structure which when cut across and held towards the light shows an iridescence. Moss agate, light-gray to white and translucent to opaque agates, display black tracings like fine moss or trees. Mocha or tree agates are covered with black, brown, or red figures, as of trees and plants. Beckite or silicified coral shells, silicified wood, wood agate, wood opal, cloudy agate, and agate jasper are some of the many varieties of this class. The common carnelians, blood-stones, and onyxes are usually counted among the agates. Uruguay, Brazil, Oberstein, Silesia, Surinam, India, Arabia, Saxony, Scotland, the United States, and Canada are the principal places where agates are found. ONYX OR AGATE ONYX. Onyx is a variety of chalcedony in bands or strata of white, gray, and black, translucent to opaque, and generally found where agates abound.