Mineral identification relies on a suite of physical characteristics determined by a mineral’s internal atomic arrangement and chemical makeup. A mineral is formally defined as a naturally occurring, inorganic solid with a definite chemical composition and an ordered internal crystalline structure. These fundamental qualities determine the unique set of external traits—like how it reflects light or how easily it can be scratched—used to distinguish one mineral from others. Since complex chemical testing is impractical in the field, observing a combination of these physical properties allows for accurate identification.
Color and Luster: The First Impressions
Color is the most immediate visual property, yet it is often the least reliable for precise identification because it is highly variable. Idiochromatic minerals are “self-colored,” meaning their color comes from an element essential to their chemical formula, such as the copper that colors malachite green. In contrast, allochromatic minerals are “other-colored,” displaying a wide range of hues due to trace impurities or defects in the crystal lattice. For example, pure quartz is colorless, but trace amounts of iron or aluminum can cause it to appear purple (amethyst) or smoky gray. Color should only be used as a preliminary clue and not a definitive identifier.
Luster, the way a mineral surface reflects light, provides a more systematic visual cue than color. This property is broadly split into metallic and non-metallic categories. Minerals with metallic luster appear shiny like a polished metal, such as galena or pyrite, and are typically opaque. Non-metallic lusters are diverse and require descriptive terms. Examples include vitreous (the glassy shine of quartz), pearly (like mother-of-pearl), or dull (a non-reflective, earthy appearance). Luster is related to the mineral’s chemical bonding and its ability to absorb or reflect light.
The Streak Test and Hardness Scale
The streak test is a simple yet highly reliable method that determines the color of a mineral when it is powdered. This is performed by scraping the mineral across a piece of unglazed porcelain, known as a streak plate. The resulting powder’s color is often a more consistent indicator of the mineral’s identity than its external color, which can be altered by weathering or impurities. For example, hematite can appear silvery-gray, black, or reddish-brown, but consistently leaves a diagnostic reddish-brown streak. Minerals harder than the streak plate (about 7 on the Mohs scale) will not leave a powder streak and instead scratch the plate.
Hardness is a measure of a mineral’s resistance to scratching or abrasion. It is quantified using the Mohs Hardness Scale, an ordinal scale ranging from 1 (softest) to 10 (hardest). The test involves attempting to scratch the unknown mineral with a material of known hardness. Common reference objects used for field identification include a fingernail (about 2.5), a copper penny (about 3.5), or a piece of glass (about 5.5). If a mineral is scratched by a penny but not by a fingernail, its hardness is estimated between 2.5 and 3.5. Hardness reflects the strength of the atomic bonds within the mineral’s crystal structure.
Cleavage, Fracture, and Crystal Habit
Cleavage describes the tendency of a mineral to break smoothly along flat planes of weakness within its atomic structure. These planes exist where the bonds between atoms are relatively weaker. Cleavage is categorized by the number of directions and the angles at which the planes intersect. For example, mica exhibits basal cleavage, breaking in only one direction to form thin, flexible sheets. Other minerals may have cubic cleavage, breaking into perfect cubes like galena, or rhombohedral cleavage, resulting in slanted, blocky fragments like calcite.
Fracture describes any breakage that is not along a smooth cleavage plane, occurring when the atomic bonds are equally strong in all directions. One distinct type is conchoidal fracture, which produces smooth, curved surfaces resembling the concentric ripples of a seashell. Quartz, which lacks cleavage, typically breaks with this shell-like pattern.
Crystal habit, or crystal form, refers to the typical shape a mineral grows into when it is allowed to form without constraint. This habit is an outward expression of the internal atomic arrangement. Examples include prismatic (elongated), cubic, or tabular (flat and plate-like). It is important to distinguish habit, which is the shape of growth, from cleavage, which is the manner of breaking.
Specific Gravity and Unique Properties
Specific gravity is a unitless measure of a mineral’s density compared to the density of water. It is determined by the ratio of the mineral’s mass to the mass of an equal volume of water. While formal measurement requires specialized equipment, a qualitative assessment can be made by judging the mineral’s “heft.” Minerals with high specific gravity, such as galena or native gold, feel noticeably heavier for their size compared to common silicate minerals. This perceived weight is a reliable property because it is directly linked to the atomic weight and packing of the elements within the mineral.
Certain minerals possess unique properties that serve as definitive identifiers, though they are not universally applicable. Magnetite, an iron oxide mineral, is strongly magnetic and will attract a magnet. Carbonate minerals, such as calcite, will effervesce or fizz when a drop of diluted acid is applied, releasing carbon dioxide gas. Some minerals, like fluorite or willemite, can exhibit fluorescence, glowing visibly under ultraviolet light.