Geology

Beryl is commonly found as hexagonal prismatic crystals and has a hardness of 7.5–8. Emerald and aquamarine are familiar gemstone varieties; colors range from green and blue to yellow or colorless.

Quartz is a common, glassy mineral with a Mohs hardness of 7, allowing it to scratch ordinary glass. It frequently forms six-sided prisms with pointed terminations and breaks with a conchoidal fracture.

Calcite commonly forms rhombohedral crystals and has perfect cleavage in three directions. Its hardness is 3, and it readily fizzes in dilute hydrochloric acid—a useful identification test.

Magnesite is often white, gray, or pale yellow and may occur in compact or granular masses. It has a hardness of about 3.5–4.5 and resembles other pale carbonate minerals.

Siderite is an iron carbonate, typically brown, tan, or yellowish, with vitreous to pearly luster. It commonly forms rhombohedral crystals or granular masses and can weather to rusty iron oxides.

Rhodochrosite is recognized by its pink to rose-red color, often with pale banding. This manganese carbonate has a hardness of about 3.5–4 and commonly occurs in rhombohedral crystals or banded masses.

Dolomite is a carbonate that commonly forms rhombohedral crystals, sometimes with curved faces. White, gray, and pink varieties occur; powdered dolomite reacts more readily with dilute acid than an intact crystal.

Apatite commonly forms hexagonal prisms with vitreous luster and has Mohs hardness 5. Green, blue, yellow, and brown varieties occur, and apatite is a useful reference mineral on the Mohs scale.

Corundum is a very hard aluminum oxide (Mohs 9), commonly forming barrel-shaped or tabular crystals. Ruby is the red gemstone variety; sapphire refers to other gem colors.

Ilmenite is typically black to brownish black, opaque, and metallic to submetallic. Its streak is usually black to reddish brown; it is an important titanium-bearing mineral.

Hematite may appear metallic gray to black or earthy red, but its diagnostic streak is reddish brown. It is a major iron ore mineral and occurs in both massive and well-crystallized forms.

Graphite is dark gray to black, soft, and greasy-feeling, and it readily leaves marks on paper or fingers. It consists of carbon arranged in sheets and often has a metallic to dull luster.

Cinnabar is a mercury sulfide noted for its vivid red to scarlet color and red streak. It is relatively soft and may occur as coatings or crystals; avoid creating dust or handling it unnecessarily.

Molybdenite has a silvery gray metallic appearance and a soft, flaky habit reminiscent of graphite. Its streak may be bluish gray, and it commonly occurs as thin plates or scales.

Almandine is an iron-rich garnet, typically deep red to reddish brown, often forming equant dodecahedral crystals. Like other garnets it lacks prominent cleavage and is comparatively hard.

Spessartine is a manganese-rich garnet often orange, reddish orange, or brownish red. It commonly forms well-defined equant crystals and has a hardness of roughly 6.5–7.5.

Uvarovite is a chromium-rich garnet notable for its emerald-green color. It often forms small, sparkling crystals coating other rocks rather than large individual crystals.

Grossular is a calcium-aluminum garnet occurring in green, yellow, pink, orange, or reddish varieties. It has vitreous luster and commonly forms equant crystals with no obvious cleavage.

Sodalite is typically rich blue, often mottled with white, although gray and other colors occur. It is a feldspathoid mineral, commonly found as massive material and distinguished from lapis lazuli by its composition and appearance.

Magnetite is a dense black iron oxide distinguished by strong magnetic attraction and a black streak. It occurs as massive grains or well-formed octahedral crystals.

Chromite is usually black to brownish black, opaque, and submetallic, often occurring as small granular crystals. It is the principal chromium ore mineral and commonly occurs in mafic and ultramafic rocks.

Halite is common rock salt, typically colorless or white, with cubic crystals and perfect cubic cleavage. It is soft (Mohs 2–2.5) and readily dissolves in water; identification does not require tasting specimens.

Sylvite is a potassium chloride mineral that may appear colorless, white, reddish, or pink. It forms cubic crystals and resembles halite, but its chemistry and associations differ; do not taste samples.

Fluorite commonly forms cubes and displays a wide range of colors, especially purple and green. It has Mohs hardness 4, perfect octahedral cleavage, and some specimens fluoresce under ultraviolet light.

Native copper has a distinctive metallic copper-red color that develops a darker patina on exposure. It may form branching or wirelike masses and is relatively soft and malleable.

Native silver is a metallic, silver-white element that often tarnishes gray or black. It occurs as wires, branching aggregates, or irregular masses and is malleable rather than brittle.

Pyrite has a brass-yellow metallic luster and often forms cubes with striated faces. Known as fool's gold, it is harder and more brittle than native gold and usually produces a dark streak.

Sphalerite is a zinc sulfide ranging from yellow or reddish brown to nearly black. It has resinous to adamantine luster, prominent cleavage, and commonly occurs with galena and other sulfide minerals.

Galena is a heavy lead-gray metallic mineral with perfect cubic cleavage. Its high density, bright metallic faces, and soft hardness (about 2.5) are useful for recognition; wash hands after handling.

Zeolites are a group of hydrated aluminosilicate minerals rather than a single mineral species. Members such as stilbite and laumontite commonly form crystals or cavity fillings in volcanic rocks.

Chalcedony is a microcrystalline variety of silica with waxy luster and conchoidal fracture. It includes varieties such as agate and some jasper; individual quartz crystals are usually too small to see.

Staurolite is a brown to reddish-brown metamorphic mineral known for cross-shaped penetration twins. It is relatively hard and often occurs in schists associated with garnet and mica.

Clinozoisite is commonly pale green, yellow-green, or nearly colorless, with a vitreous luster. It occurs in metamorphic and altered rocks and is often recognized by its prismatic habit and high relief in thin section.

Diopside is a pyroxene, commonly pale green, dark green, or nearly colorless. It has a hardness around 5.5–6.5 and two cleavage directions intersecting at nearly right angles.

Hornblende is the name commonly used for dark amphiboles that form elongate, prismatic crystals. Black to dark green specimens show two prominent cleavage directions meeting at approximately 56° and 124°.

Glaucophane is a blue to blue-gray amphibole that commonly occurs in blueschist metamorphic rocks. Its distinctive blue color and elongated crystals help separate it from many other amphiboles.

Chrysotile is a serpentine-group mineral typically occurring as fine, flexible fibers. It is a form of asbestos; do not break, abrade, or disturb fibrous specimens.

Antigorite is a serpentine-group mineral commonly found as platy, foliated, or scaly aggregates in altered ultramafic rocks. Colors are typically green to yellow-green, with a waxy or silky appearance.

Chlorite refers to a mineral group, commonly green and occurring as soft, flaky, or platy aggregates. Chlorite minerals are frequent alteration and low-grade metamorphic products and may resemble green mica.

Muscovite is a light-colored mica that readily splits into thin, flexible, elastic sheets. It has perfect basal cleavage and commonly appears silvery, colorless, or pale tan with a pearly luster.

Biotite is a dark mica commonly appearing black or dark brown. It splits into thin, elastic sheets along perfect basal cleavage and is common in igneous and metamorphic rocks.

Lepidolite is a lithium-rich mica-group name often associated with lilac, pink, or lavender colors. It forms platy or scaly aggregates and commonly occurs in lithium-bearing pegmatites.

Orthoclase and microcline are potassium feldspars, often white, cream, or salmon pink. Both have hardness near 6 and two nearly perpendicular cleavage directions; microcline may show distinctive cross-hatched twinning in thin section.

Malachite is a bright green copper carbonate hydroxide, often forming banded, fibrous, or botryoidal masses. Its green color and streak are characteristic; it commonly occurs in oxidized copper deposits.

Azurite is a deep blue copper carbonate hydroxide that may form brilliant blue crystals or earthy masses. It often occurs alongside green malachite and can alter to malachite near the surface.

Gypsum is a very soft calcium sulfate mineral (Mohs 2) that can be scratched by a fingernail. Varieties include clear selenite crystals, fibrous satin spar, and fine-grained alabaster.

Pyrrhotite is an iron sulfide generally bronze-brown to brownish gray with metallic luster. Some specimens are magnetic, but magnetic strength varies with composition and structure.

Realgar is an orange-red arsenic sulfide mineral often associated with orpiment. It is light-sensitive and toxic; specimens should be stored away from light and handled with care.

Orpiment is an arsenic sulfide distinguished by its bright yellow to golden-orange color and soft, pearly to resinous appearance. It is toxic and should not be abraded or handled carelessly.

Limonite is a traditional field term for earthy yellow-brown to rusty iron oxides and oxyhydroxides, rather than a single mineral species. It often forms coatings, nodules, or weathered masses.

Opal is hydrated amorphous silica and is classified as a mineraloid rather than a crystalline mineral. Common opal may be dull or glassy, while precious opal displays play-of-color.

Fayalite is the iron-rich end member of the olivine series, typically dark green, brown, or black. It occurs in certain iron-rich igneous rocks and metamorphosed iron-rich settings.

Andalusite is an aluminosilicate commonly formed during metamorphism of clay-rich rocks. Some crystals show a characteristic dark cross in section; this variety is called chiastolite.

Sillimanite is an aluminosilicate found in high-grade metamorphic rocks. It commonly forms slender prisms or fibrous aggregates and may appear colorless, white, brown, or gray.

Topaz is a hard mineral (Mohs 8) commonly occurring as glassy prismatic crystals. Natural colors include colorless, pale yellow, blue, and brown; it has perfect cleavage in one direction.

Hemimorphite is a zinc silicate that commonly forms white, pale blue, or colorless crusts and radiating crystal aggregates. Well-formed crystals are known for different terminations at opposite ends.

Cordierite commonly appears blue-gray to violet-blue or nearly colorless and occurs in metamorphic rocks. Some specimens display color changes when viewed from different directions, a property called pleochroism.

Enstatite is a magnesium-rich orthopyroxene, commonly gray, brown, or green. It may form prismatic crystals with the two characteristic pyroxene cleavage directions close to 90°.

Aragonite is a polymorph of calcium carbonate that commonly forms needlelike, branching, or radiating crystals. It has the same chemistry as calcite but a different crystal structure.

Barite is a relatively heavy sulfate mineral, commonly white, colorless, or pale blue. It forms tabular crystals and can produce rosette-shaped aggregates sometimes called desert roses.

Celestite is a strontium sulfate mineral often forming pale blue, colorless, or white crystals. It commonly occurs in sedimentary rocks and can form attractive prismatic or tabular crystals.

Native sulfur is usually bright yellow with resinous luster and relatively low hardness. It occurs in volcanic environments and evaporite-related deposits; odor alone is not a reliable identification test.

Enargite is an opaque copper arsenic sulfide, typically gray-black to iron-black, with metallic luster. It commonly occurs in hydrothermal ore deposits; dust from arsenic-bearing specimens should be avoided.

Zircon often forms small, well-shaped tetragonal prismatic crystals with pyramidal terminations. Colors vary widely; its high hardness and resistance to weathering make it useful in geologic dating.

Vesuvianite commonly forms short to long prismatic crystals, often brown, green, yellow, or reddish. It is commonly associated with contact-metamorphosed carbonate rocks and calc-silicate assemblages.

Rutile is a titanium dioxide mineral commonly occurring as reddish-brown, black, or golden needlelike crystals. Slender rutile needles may appear as inclusions in quartz.

Chalcopyrite is a brass-yellow copper iron sulfide with metallic luster, frequently showing iridescent tarnish. It is softer than pyrite and may have a deeper yellow color and less sharply cubic habit.

Bornite is a copper iron sulfide often called peacock ore because its tarnished surface can show purple, blue, and iridescent colors. Fresh surfaces are commonly brownish bronze.

Kyanite is an aluminosilicate often seen as blue, bladed crystals in metamorphic rocks. Its hardness varies with the direction of scratching, a notably unusual property.

Tourmaline is a mineral group commonly occurring as elongated, vertically striated prisms with roughly triangular cross sections. Black schorl is common, while other varieties may be pink, green, or multicolored.

Wollastonite is a calcium silicate that may form white, gray, or pale pink bladed or fibrous aggregates. It commonly occurs in metamorphosed limestone and other calcium-rich rocks.

Rhodonite is a pink to rose-red manganese silicate often marked by black manganese-oxide veins. It is harder than rhodochrosite and is commonly used as an ornamental stone.

Kaolinite is a fine-grained clay mineral commonly white, cream, or pale tan. It forms through alteration of feldspars and other aluminum-bearing silicates and has a soft, earthy appearance.

Talc is the softest reference mineral on the Mohs scale (hardness 1), with a slippery or soapy feel. It commonly occurs in foliated or massive pale green, white, or gray aggregates.

Albite is a sodium-rich plagioclase feldspar, usually white or colorless, with hardness around 6. It commonly displays fine parallel striations from polysynthetic twinning on cleavage surfaces.

Anorthite is the calcium-rich end member of the plagioclase feldspar series. It is commonly white to gray and occurs in calcium-rich igneous and metamorphic rocks.

Labradorite is a plagioclase feldspar known for iridescent flashes of blue, green, or gold called labradorescence. It commonly occurs in mafic igneous rocks and has hardness near 6.

Spodumene is a lithium-bearing pyroxene commonly forming elongated, glassy crystals in pegmatites. Gem varieties include green hiddenite and pink to lilac kunzite; it has hardness about 6.5–7.