About the Varieties and Properties of Moss Agate

MOSS AGATE is a variety of chalcedonic quartz that has some vogue in the jewelry of to-day, and is one of the most interesting features of gem mineralogy. Enclosed in this stone are what seem to be long hairs and fibres, usually irregularly interwoven, and having the effect of various species of moss. These branching forms, so imitative of one of the most beautiful of plants, are manganese or iron oxide, and not imprisoned vegetation, or prehistoric insects which really were imprisoned in amber, and have been preserved through ages to furnish food for speculation for latter-day naturalists.

The name agate is derived from the river Achates, in Sicily, now called the Drillo, in the Val de Noto. Theophrastus states that this is where ancient agates were found.

Moss agates and Mocha stones are varieties of crypto-crystalline (obscurely crystalline) quartz of fibrous structure, and are slightly softer and lighter than crystallised quartz. The hardness of crypto-crystalline quartz is 6.5; specific gravity, 2.6; it is more difficult to break than crystalline quartz, being very tough, which makes these varieties--their principal differences being of colour and colour-pattern--eminently fit for carving.

The finest moss agates known to-day come from India, and those specimens called "mocha stone" originally came, it is believed, from the vicinity of Mocha, an Arabian seaport at the entrance of the Red Sea most famous for its aromatic coffee. The Oriental moss agates are common in the volcanic rocks (trap rock) of western India, occurring with Mocha stone. Blocks weighing as high as thirty pounds have been obtained. It occurs also as pebbles in many Indian rivers. From China has come, during recent years, a supply of natural green and artificial yellow and red moss agates, which have, to a considerable extent, replaced others on the market. Fine moss agates are abundant in various parts of the Rocky Mountains; the best are found in the form of rolled pebbles in the beds of streams. As souvenirs, and for sentimental reasons of local interest, these beautiful gem stones of our Rocky Mountain States are cut and mounted; in the tourist the Western jeweller and curio-dealer finds for these American moss agates a good customer.

Mocha stone ("tree stone" or dendritic agate) is a white or grey chalcedony showing brown, red, or black dendritic markings resembling trees and plants. These have been formed by the percolation of a solution containing iron or manganese through the fine fissures of the stone, and the subsequent deposition of the colouring matter originally held in solution. The brown and red markings are caused by oxide of iron, and the black by oxide of manganese.