Minerals have definite crystalline structures and chemical compositions that give them unique sets of physical and chemical properties shared by all specimens of that mineral, regardless of when or where they formed. For example, two samples of the mineral quartz will be equally hard and equally dense, and they will break in a similar manner. However, the physical properties of individual samples may vary within specific limits due to ionic substitutions, inclusions of foreign elements (impurities), and defects in the crystalline structure.

Some mineral properties, called diagnostic properties, are particularly useful in identifying an unknown mineral. The mineral halite, for example, has a salty taste. Because so few minerals share this property, a salty taste is considered a diagnostic property of halite. Other properties of certain minerals, particularly color, vary among different specimens of the same mineral. These properties are referred to as ambiguous properties.

Optical Characteristics

Of the many diagnostic properties of minerals, their optical (visual) characteristics, such as luster, streak, and ability to transmit light are most frequently used for mineral identification. The optical characteristic of color, however, is an ambiguous property for many minerals.

Luster

The appearance or quality of light reflected from the surface of a mineral is known as luster. Minerals that are shiny like a metal, regardless of color, are said to have a metallic luster ▼. Some metallic minerals, such as native copper and galena, develop a dull coating or tarnish when exposed to the atmosphere. Because they are not as shiny as samples with freshly broken surfaces, these samples are often said to exhibit a submetallic luster ▼.

Most minerals have a nonmetallic luster and are described using various adjectives. For example, some nonmetallic minerals are described as being vitreous, or glassy. Other nonmetallic minerals are described as having a dull, or earthy, luster (a dull appearance like soil) or a pearly luster (such as a pearl or the inside of a clamshell). Still others exhibit a silky luster (like satin cloth) or a greasy luster (as though coated in oil).

Color 

Although color is generally the most conspicuous characteristic of any mineral, it is considered a diagnostic property of only a few minerals. Slight impurities in the common minerals fluorite and quartz, for example, give them a variety of tints, including pink, purple, yellow, white, gray, and even black ▼. Other minerals, such as tourmaline, also exhibit a variety of hues, with multiple colors sometimes occurring in the same sample. Thus, the use of color as a means of identification is often ambiguous or even misleading.

Streak

The color of a mineral in powdered form, called streak, is often useful in identification. A mineral’s streak is obtained by rubbing it across a streak plate (a piece of unglazed porcelain) and observing the color of the mark it leaves ▼. Although a mineral’s color may vary from sample to sample, its streak is usually consistent in color. (Note that not all minerals produce a streak when rubbed across a streak plate. Quartz, for example, is harder than a porcelain streak plate and therefore leaves no streak.)

Streak can also help distinguish between minerals with metallic luster and those with nonmetallic luster. Metallic minerals generally have a dense, dark streak, whereas minerals with nonmetallic luster typically have a light-colored streak.

Ability to Transmit Light

Another optical property used to identify minerals is the ability to transmit light. When no light is transmitted through a mineral sample, that mineral is described as opaque; when light, but not an image, is transmitted, the mineral is said to be translucent. When both light and an image are visible through the sample, the mineral is described as transparent.

Crystal Shape, or Habit

Mineralogists use the term crystal shape, or habit, to refer to the common or characteristic growth pattern of individual crystals or aggregates of crystals. Some minerals tend to grow equally in all three dimensions, whereas others tend to be elongated in one direction or flattened if growth in one dimension is suppressed. The crystals of a few minerals can have a regular polygonal shape that is helpful in identification. For example, magnetite crystals sometimes occur as octahedrons (with 12 sides), garnets often form dodecahedrons (with 8 sides), and halite and fluorite crystals tend to grow as cubes or near-cubes. Most minerals have just one common crystal shape, but a few, such as the pyrite samples shown in ▼, have two or more characteristic crystal shapes.

In addition, some mineral samples consist of numerous intergrown crystals exhibiting characteristic shapes that are useful for identification. Terms commonly used to describe these and other crystal habits include equant (equidimensional), bladed, fibrous, tabular, cubic, prismatic, platy, blocky, and banded. Some of these habits are pictured in ▼.

Mineral Strength

How easily minerals break or deform under stress is determined by the type and strength of the chemical bonds that hold the crystals together. Mineralogists use terms including hardness, cleavage, fracture, and tenacity to describe mineral strength and how minerals break when stress is applied.

Hardness

One of the most useful diagnostic properties of minerals is their hardness, a measure of the resistance of a mineral to abrasion or scratching. This property is determined by rubbing a mineral of unknown hardness against one of known hardness or vice versa. A numerical value of hardness can be obtained by using the Mohs scale of hardness, which consists of 10 minerals arranged in order from 1 (softest) to 10 (hardest), as shown in ▼. It should be noted that the Mohs scale is a relative ranking and does not imply that a mineral with a hardness of 2, such as gypsum, is twice as hard as mineral with a hardness of 1, like talc. In fact, gypsum is only slightly harder than talc, as ▼ indicates.

In the laboratory, common objects used to determine the hardness of a mineral can include a human fingernail, which has a hardness of about 2.5, a copper penny (3.5), and a piece of glass (5.5). The mineral gypsum, which has a hardness of 2, can be easily scratched with a fingernail. On the other hand, the mineral calcite, which has a hardness of 3, will scratch a fingernail but will not scratch glass. Quartz, one of the hardest common minerals, will easily scratch glass. Diamonds, hardest of all, will scratch anything, including other diamonds.

Cleavage

In the crystal structure of many minerals, some atomic bonds are weaker than others. It is along these weak bonds that minerals tend to break when they are stressed. The diagnostic property of cleavage (from German kleiben, “to cleave, split”) is the tendency of a mineral to break (cleave) along planes of weak bonding. Not all minerals have cleavage, but those that do can be identified by the relatively smooth, flat surfaces that are produced when the mineral is broken.

The simplest type of cleavage is exhibited by the micas ▼). Because these minerals have very weak bonds in one direction, they cleave to form thin, flat sheets. 

Some minerals have excellent cleavage in one, two, three, or more directions, whereas others exhibit fair or poor cleavage, and still others have no cleavage at all. When minerals break evenly in more than one direction, cleavage is described by the number of cleavage directions and the angle(s) at which they meet ▼.

Each cleavage surface that has a different orientation is counted as a different direction of cleavage. For example, some minerals, such as halite, cleave to form six-sided cubes. Because a cube is defined by three different sets of parallel planes that intersect at 90-degree angles, cleavage for the mineral halite is described as three directions of cleavage that meet at 90 degrees.

The difference between crystal shape and cleavage can be confusing; remember that cleavage is how a mineral breaks, and crystal shape is how a crystal grows (forms) naturally. When a mineral exhibits cleavage, it breaks into pieces that all have the same geometry. By contrast, the smooth-sided quartz crystals shown in the figure below ▼ do not have cleavage. If broken, they fracture into shapes that do not resemble one another or the original crystals.

Fracture 

Minerals having chemical bonds that are equally, or nearly equally, strong in all directions exhibit a property called fracture ▼. When minerals fracture, most produce uneven surfaces and are described as exhibiting irregular fracture. Irregular fracture in itself is more of an ambiguous property. 

However, some minerals, including quartz, sometimes break into smooth, curved surfaces resembling broken glass. Such breaks are called conchoidal fractures ▼. Still other minerals exhibit fractures that produce splinters or fibers referred to as splintery fracture and fibrous fracture, respectively.

Tenacity

The term tenacity describes how a mineral responds to stress—for instance, whether it tends to break in a brittle fashion or bend elastically. As mentioned earlier, nonmetallic minerals, such as quartz, and minerals that are ionically bonded, such as fluorite and halite, tend to be brittle and fracture or exhibit cleavage when struck. By contrast, native metals, such as copper and gold, are malleable, which means they can be hammered without breaking. In addition, minerals that can be cut into thin shavings, including gypsum and talc, are described as sectile. Still others, notably the micas, are elastic and bend and snap back to their original shape after stress is released. Taken together with other properties, tenacity can be a helpful diagnostic tool for identifying a mineral.

Density, an important property of matter, is defined as mass per unit volume. Mineralogists often use a related measure called specific gravity to describe the density of minerals. Specific gravity is a number representing the ratio of a mineral’s weight to the weight of an equal volume of water.

Most common minerals have a specific gravity between 2 and 3. For example, quartz has a specific gravity of 2.65. By contrast, some metallic minerals, such as pyrite, native copper, and magnetite, are more than twice as dense and thus have more than twice the specific gravity of quartz. Galena, an ore from which lead is extracted, has a specific gravity of roughly 7.5, whereas 24-karat gold has a specific gravity of approximately 20. For these minerals, specific gravity is a good diagnostic property when taken into account with other properties.

With a little practice, you can estimate the specific gravity of a mineral by hefting it in your hand. Does this mineral feel about as “heavy” as similarly sized rocks or minerals you have handled? If the answer is “yes,” the specific gravity of the sample will likely be between 2.5 and 3.

Other Properties of Minerals

In addition to the properties discussed thus far, some minerals can be recognized by other diagnostic properties. As mentioned previously, halite is ordinary table salt, so it can be quickly identified through taste. Talc and graphite both have distinctive feels: Talc feels soapy, and graphite feels greasy. Further, the streaks of many sulfur-bearing minerals smell like rotten eggs. A few minerals, such as magnetite, have high iron content and can be picked up with a magnet, while some varieties (such as lodestone) are themselves natural magnets and will pick up small iron-based objects, such as pins and paper clips.

Moreover, some minerals exhibit special diagnostic optical properties. For example, when a transparent piece of calcite is placed over printed text, the letters appear twice. This optical property is known as double refraction ▼.

One very simple diagnostic chemical test to detect carbonate minerals involves placing a drop of dilute hydrochloric acid from a dropper bottle onto a freshly broken mineral surface. Samples containing carbonate minerals will effervesce (fizz) as carbon dioxide gas is released ▼. This test is especially useful in identifying calcite, a common carbonate mineral.

• The composition and internal crystalline structure of a mineral give it specific physical properties. Mineral properties useful in identifying minerals are termed diagnostic properties.

• Luster is a mineral’s ability to reflect light. The terms transparent, translucent, and opaque describe the degree to which a mineral can transmit light. Color can be unreliable for mineral identification, as impurities can “stain” minerals with diverse colors. A more reliable identifier is streak, the color of the powder generated by scraping a mineral against a porcelain streak plate.

• Crystal shape, also called crystal habit, is often useful for mineral identification and represents the growth pattern of a mineral.

• Variations in the strength of chemical bonds give minerals diagnostic properties, such as hardness (resistance to being scratched) and tenacity (response to deforming stress, such as whether the mineral tends to undergo brittle breakage like quartz, bend elastically like mica, or deform malleably like gold). Cleavage, the preferential breakage of a mineral along planes of weakly bonded atoms, is very useful in identifying minerals.

• The amount of matter packed into a given volume determines a mineral’s density, a diagnostic property. To compare the densities of minerals, mineralogists use a related quantity, known as specific gravity, which is the ratio between a mineral’s density and the density of water.

• Other properties are diagnostic for certain minerals but rare in most others; examples include smell, taste, feel, reaction to hydrochloric acid, magnetism, and double refraction.

• ambiguous properties: Properties of minerals that are not diagnostic because they vary among different specimens of the same mineral.

• The tendency of a mineral to break along planes of weak bonding.cleavage: 

• color: A mostly ambiguous mineral property describing the appearance of a substance as a result of how it reflects or emits light.

• crystal shape: Refers to the common or characteristic shape of a mineral crystal or aggregate of crystals. Also called habit.

• density: The mass per unit volume of a substance, usually expressed as grams per cubic centimeter.

• diagnostic properties: Properties of minerals that aid in its identification. Taste, feel, crystal shape, and streak are examples of diagnostic properties.

• fracture: In reference to minerals properties, the way a mineral breaks into uneven surfaces rather than along cleavage planes. When referring to masses of rock, any break or rupture in rock along which no appreciable movement has taken place.

• habit: Refers to the common or characteristic shape of a mineral crystal or aggregate of crystals. Also called crystal shape.

• hardness: The resistance of a mineral  to scratching.

• luster: The appearance or quality of light reflected from the surface of a mineral.

• Mohs scale: A relative ranking of 10 minerals used as a standard in determining hardness, where the minerals are arranged from softest (1) to hardest (10).

• specific gravity: The ratio of a substance’s weight to the weight of an equal volume of water.

• streak: The color of a mineral in powdered form, often obtained by rubbing the mineral across an unglazed piece of porcelain.

• tenacity: A mineral’s response to stress or its resistance to breaking or deforming.