Brown is a common color for rocks in general at the Earth's surface. It may take careful observation to evaluate a brown mineral, and color may be the least important thing to see. Moreover, brown is a mongrel color that blends into red, green, yellow, white and black. Look at a brown mineral in good light, making sure to inspect a fresh surface, and ask yourself exactly what kind of brown it is. Determine the mineral's luster and be ready to do hardness tests, too. Finally, know something about the rock that the mineral occurs in. Here are the most common possibilities. The first four—clays, two iron oxide minerals, and sulfides—account for nearly all occurrences; the rest are presented in alphabetical order.
Clays
Shale or claystone — Geology Guide photo
Clay is a set of minerals with microscopic grains and colors ranging from medium brown to white. It's the main ingredient of shale. It never forms visible crystals. Geologists often nibble on shale; pure clay is a smooth substance with no grittiness on the teeth. Luster dull; hardness 1 or 2.
Hematite
Botryoidal hematite — Geology Guide photo
The most common iron oxide, hematite ranges from red and earthy, through brown, to black and crystalline. In every form it takes, hematite has a red streak. It may also be slightly magnetic. Suspect it wherever a brown-black mineral appears in sedimentary or low-grade metasedimentary rocks. Luster dull to semimetallic; hardness 1 to 6.
Goethite
Goethite — Geology Guide photo
Goethite is fairly common, but seldom concentrated in bulk form. It's much harder than clay, has a yellowish brown streak and is well developed where iron minerals have weathered. "Bog iron" is typically goethite. Luster dull to semimetallic; hardness around 5.
Sulfide Minerals
Chalcopyrite — Geology Guide photo
Some of the metal sulfide minerals are typically bronze to brown (pentlandite, pyrrhotite, bornite). Suspect one of these if it occurs along with pyrite or other common sulfides. Luster metallic; hardness 3 or 4.
Amber
Amber — Mersey Viking (Flickr CC BY-NC-SA 2.0)
A fossil tree resin rather than a true mineral, amber is restricted to certain mudstones and ranges in color from honey to the dark brown of bottle glass. It's lightweight, like plastic, and it often contains bubbles, sometimes fossils like insects. It will melt and burn in a flame. Luster resinous; hardness less than 3.
Andalusite
Andalusite — -Merce- (Flickr CC BY-NC-SA 2.0)
A sign of high-temperature metamorphism, andalusite may be pink or green, even white, as well as brown. It usually occurs in stubby crystals in schist, with square cross sections that may display a crosslike pattern (chiastolite). Luster glassy; hardness 7.5.
Axinite
Axinite — Geology Guide photo
This odd boron-bearing silicate mineral is more readily found in rock shops than in the field, but you might see it in metamorphic rocks near granite intrusions. Its lilac-brown color and flat bladed crystals with striations are distinctive. Luster glassy; hardness around 7.
Cassiterite
Cassiterite — Wikimedia Commons
An oxide of tin, cassiterite occurs in high-temperature veins and pegmatites. Its brown color shades into yellow and black. Even so, its streak is white, and it will feel heavy if you can get a big enough piece to heft in your hand. Its crystals, when broken, typically show bands of color. Luster adamantine to greasy; hardness 6–7.
Copper
Wire Copper — Geology Guide photo
Copper may be reddish brown due to impurities. It occurs in metamorphic rocks and in hydrothermal veins near volcanic intrusions. Copper should bend like the metal it is, and it has a distinctive streak. Luster metallic; hardness 3.
Corundum
Corundum — Geology Guide photo
Its extreme hardness is the surest sign of corundum, along with its occurrence in high-grade metamorphic rocks and pegmatites in six-sided crystals. Its color ranges widely around brown and includes the gemstones sapphire and ruby. Rough cigar-shaped crystals are available in any rock shop. Luster adamantine; hardness 9.
