How to Distinguish Brown Minerals

Clays

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

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 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

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

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

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

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

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

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

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

Garnets

The common garnet minerals may appear brown in addition to their usual colors. The six main garnet minerals vary in their typical geologic settings, but all have the classic garnet crystal shape, a round dodecahedron. Brown garnets may be spessartine, almandine, grossular or andradite depending on the setting.

• Luster: Glassy
• Hardness: 6-7.5

Monazite

This rare-earth phosphate is uncommon but widespread in pegmatites as flat, opaque crystals that break into splinters. Its color tends toward reddish-brown. Because of its hardness, monazite may persist in sands, and the rare-earth metals were once mined from sand deposits.

• Luster: Adamantine to resinous
• Hardness: 5

Phlogopite

A brown mica mineral that is basically biotite without the iron, phlogopite favors marble and serpentinite. One key feature it may display is asterism when you hold a thin sheet against a light.

• Luster: Pearly or metallic
• Hardness: 2.5-3

Pyroxenes

While the most common pyroxene mineral, augite, is black, the diopside and enstatite series are shades of green that may veer over to brown with high iron contents. Look for bronze-colored enstatite in igneous rocks and brown diopside in metamorphosed dolomite rocks.

• Luster: Glassy
• Hardness: 5-6

Quartz

Brown crystalline quartz may be called cairngorm; its color arises from missing electrons (holes) plus aluminum impurities. The gray variety called smoky quartz or morion is more common. Quartz is usually easy to identify by its typical hexagonal spears with grooved sides and conchoidal fracture.

• Luster: Glassy
• Hardness: 7

Siderite

A brown mineral occurring in carbonate ore veins is usually siderite, iron carbonate. It also may be found in concretions, and sometimes in pegmatites. It has the typical appearance and rhombohedral cleavage of carbonate minerals.

• Luster: Glassy to pearly
• Hardness: 3.5-4

Sphalerite

Sulfide ore veins in rocks of all types are the typical home of this zinc mineral. Its iron content gives sphalerite a color range of yellow through red-brown to black. It may form chunky crystals or granular masses. Look for galena and pyrite with it.

• Luster: Adamantine to resinous
• Hardness: 3.5-4

Staurolite

Perhaps the easiest brown crystalline mineral to learn, staurolite is a silicate found in schist and gneiss as isolated or twinned crystals ("fairy crosses.") Its hardness will distinguish it if there's any doubt. Found in any rock shop, too.

• Luster: Glassy
• Hardness: 7-7.5

Topaz

This familiar rock-shop item and gemstone may be seen in pegmatites, high-temperature veins and in rhyolite flows where its clear crystals line gas pockets. Its brown color is light and tends toward yellow or pink. Its great hardness and perfect basal cleavage are clinchers.

• Luster: Glassy
• Hardness: 8

Zircon

A few small zircon crystals are found in many granites and sometimes in marble and pegmatites. Geologists prize zircon for its use in dating rocks and studying early Earth history. Although zircon gemstones are clear, most zircon in the field is dark brown. Look for bipyramidal crystals or short prisms with pyramidal ends.

• Luster: Adamantine or glassy
• Hardness: 6.5-7.5

Other Minerals

Brown is an occasional color for many minerals, whether they are typically green (apatite, epidote, olivine, pyromorphite, serpentine) or white (barite, calcite, celestine, gypsum, heulandite, nepheline) or black (biotite) or red (​cinnabar, eudialyte) or other colors (hemimorphite, mimetite, scapolite, spinel, wulfenite.) Observe which way the brown color tends, and try one of those possibilities.​