Brown Minerals

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.

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.

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.

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.

Brown crystalline quartz may be called cairngorm; its color arises from missing electrons (holes) plus aluminum impurities. More common is the gray variety called smoky quartz or morion. Usually quartz is easy to tell by its typical hexagonal spears with grooved sides and conchoidal fracture. Luster glassy; hardness 7.

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.

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.

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.

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.

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.

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.