1911 Encyclopædia Britannica/Mocha Stone

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MOCHA STONE, a name applied to chalcedony with dendritic markings, said to have been obtained originally from Mocha in Arabia. The markings which sometimes simulate with curious fidelity the form of miniature trees and shrubs, are caused by the infiltration of solutions carrying iron and manganese, which are deposited as thin films of oxide along the cracks of the stone, producing black, brown or red dendrites, effectively disposed on a ground of grey or white chalcedony. Most of the Mocha stones of commerce are obtained from India, where they are found among the agate-pebbles resulting from the disintegration of the trap rocks of the Deccan. In recent years the formation of dendrites has been artificially effected at the agate-works of Oberstein, so as to imitate the true Mocha stones.