moss agate

moss agate

noun
a kind of agate or chalcedony containing brown or black mosslike dendritic markings from various impurities.

Origin:
1790–1800

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World English Dictionary
moss agate
 
n
a variety of chalcedony with dark greenish mossy markings, used as a gemstone

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Encyclopedia Britannica
Encyclopedia

moss agate

grayish to milky-white agate (q.v.), a variety of the silica mineral quartz that contains opaque, dark-coloured inclusions whose branching forms resemble ferns, moss, or other vegetation. The included materials, mainly manganese and iron oxides, are of inorganic origin. Most moss agates are found as fragments weathered from volcanic rocks. Long used for ornamental purposes, they are obtained chiefly from India, Brazil, Uruguay, central Europe, and the western United States. The best stones are cut in flat or rounded form, and some are dyed to improve their colour. Its properties are those of quartz (see silica mineral [table]).

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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