patina

patina (păt`ənə), coating of carbonate of copper on articles of copper or bronze, formed after long exposure to a moist atmosphere or burial in the earth. Although commonly green, patina varies in color and consistency; it may be red, brown, black, blue, or gray, or it may be smooth, glossy, or crusty. It may be imitated by a number of oxidation processes. The term has been extended to include the film formed on metals, pottery, marble, and other materials by exposure and to the mellow surface acquired by furniture with time and waxing.

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desert varnish

 or patina

Thin, dark red to black mineral coating (generally iron and manganese oxides and silica) deposited on pebbles and rocks on the surface of desert regions. As dew and soil moisture brought to the surface by capillary action evaporate, their dissolved minerals are deposited on the surface. Wind abrasion removes the softer salts and polishes the surface to a glossy finish. Both high evaporation rates and sufficient precipitation are necessary for desert varnish to form.

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patina¹

a film of oxide formed on the surface of a metal, esp the green oxidation of bronze or copper

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patina²

a broad shallow dish used in ancient Rome

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patina [′pat·ən·əorpə′tē·nə]

(geology)

A thin, colored film produced on a rock surface by weathering.

(metallurgy)

The greenish product, usually basic copper sulfate, formed on copper and copper-rich alloys as a result of prolonged atmospheric corrosion.