thumb|Color chart for shale based on oxidation state and organic carbon content Shales are typically gray in color and are composed of clay minerals and quartz grains. The addition of variable amounts of minor constituents alters the color of the rock. Red, brown and green colors are indicative of ferric oxide (hematite – reds), iron hydroxide (goethite – browns and limonite – yellow), or micaceous minerals (chlorite, biotite and illite – greens). The color shifts from reddish to greenish as iron in the oxidized (ferric) state is converted to iron in the reduced (ferrous) state. Black shale results from the presence of greater than one percent carbonaceous material and indicates a reducing environment. Pale blue to blue-green shales typically are rich in carbonate minerals.
thumb|Color chart for shale based on oxidation state and organic carbon content Shales are typically gray in color and are composed of clay minerals and quartz grains. The addition of variable amounts of minor constituents alters the color of the rock Red, brown and green colors are indicative of ferric oxide (hem.atite – reds), iron hydroxide (goethite – browns and limonite – yellow), or micaceous minerals (chlorite, biotite and illite – greens). The color shifts from reddish to greenish as iron in the oxidized (ferric) state is converted to iron in the reduced (ferrous) state. Black shale results from the presence of greater than one p.ercent carbonaceous material and indicates a reducing environment. Pale blue to blue-green shales typically are rich in carbonate minerals