A mineral, by definition, is any naturally (not manmade) occurring, inorganic (not a result of life plant or animal) substance. Its chemical structure can be exact, or can vary. All minerals belong to a chemical group, which represents their affiliation with certain elements or compounds. The classified chemical groups are known as:
Minerals also have distinctive properties, such as color, hardness, specific gravity, luster, fracture, tenacity. Many of these properties can vary among a single mineral.
Color is the easiest physical property to describe, however it can also be the most difficult property to make a mineral identification. Some minerals always have the same color, such as gold, whereas some minerals, such as quartz, fluorite, and calcite, come in all colors. The presence and intensity of certain elements determines a specimens color. Minerals which have an inherit color (minerals which always occur in the same color) have essential elements in them which cause that color. This includes azurite and malachite, which have their strong blue and green color due to their copper. But there are many minerals which have slight additions of color causing elements in some specimens that cause it to be a different color. For example, pure quartz (SiO2), is colorless, whereas amethyst (a variety of quartz), which has traces of iron in it, has a strong purple color. Rose quartz's pink color is caused by traces of titanium or manganese.
Certain minerals will exhibit a color change when exposed to light, heat, or radiation. Realgar will transform into orpiment, and orpiment will crumble into a light - yellow powder if exposed to light. Some minerals, such as proustite, vivianite, fluorite, will darken upon prolonged exposure to light, whereas other minerals, such as kunzite (spodumene) will fade.
Most secondary copper minerals show a bright blue (and sometimes green) color. Iron will usually cause a mineral to exhibit a dark red or brown, and manganese is responsible for the coloring of many pink minerals. Some minerals, such as cassiterite and zincite, have chemical structures that would cause it to be colorless, but due to impurities and other factors, they are never found colorless. Most secondary uranium minerals exhibit either a bright neon yellow or green color . Minerals containing, aluminum, sodium, and potassium, are usually colorless or very lightly colored. In some cases, a mineral's color can depend on its atomic bonding rather than composition, such is the case in the difference between diamond and graphite. They both have the same elemental makeup - Carbon (C), yet one is almost always white to very lightly colored, while the other is always grayish black.
Some minerals can tarnish, thereby affecting the color of the specimen. The best examples are silver tarnishing black, copper tarnishing green, and bornite tarnishing to a play of different colors .
Some minerals, such as opal (a variety of quartz), display a multicolored effect when viewed from different angles . This is called opalescence. A few minerals appear to change color when viewed in different light. Alexandrite is dark green in natural light, but takes on a purplish hue when seen in artificial light . Other minerals will change to a different color when viewed from different angles. This is called dichroism. Cordierite has the greatest dichroic ability of any mineral, such that a blue purplish crystal will turn gray when rotated .
Streak is the color of a minerals powder when it is crushed. Some minerals have a different color powder than their actual color. Every mineral has an inherent streak no matter what color it is. For example, calcite occurs in many different colors, shapes, and varieties. But every single variety of calcite has a white streak. Streak is useful to distinguish two minerals that have the same color, but a different streak . A fine example is distinguishing gold, which has a yellow streak, and pyrite, which has a black streak. Another example is distinguishing magnetite, which has a black streak, and hematite, which has a reddish streak.
Most light colored, nonmetallic minerals have a white or colorless streak, as do most silicates, carbonates, and most transparent minerals. The streak test is most useful for identifying dark colored minerals, especially metals. Most mineral references don't make a distinction between a white or colorless streak, since the difference is minimal. A mineral with a white or colorless streak will not leave visibly colored powder.
Hardness plays a major role in identifying a mineral. It can make the identification process much simpler by considerably narrowing a search. Hardness is defined by how well a mineral will resist scratching by another mineral. A scale to measure hardness was invented by a mineralogist named Frederick Mohs (1822) and is still the standard scale for measuring hardness. The scale consists of numbers one through ten; 1 being the softest and 10 being the hardest. Each number represents a different mineral - each harder than the previous number. The 10 minerals are:
| Scale | Mineral | common household item (with a fixed hardness) |
| 1 | Talc | |
| 2 | Gypsum | fingernail (2 ½) |
| 3 | Calcite | copper penny (3) |
| 4 | Fluorite | |
| 5 | Apatite | knife blade, glass (5½) |
| 6 | Feldspar | steel file, streak plate (6½) |
| 7 | Quartz | |
| 8 | Topaz | |
| 9 | Corundum | |
| 10 | Diamond |
All minerals are in this scale, since talc is the softest known mineral and diamond is the hardest. Suppose a mineral scratches fluorite, but not apatite, then it has a hardness between 4 and 5. Hardness is usually rounded off to the nearest half number.
Minerals can be damaged and lose value if not scratched properly. If a mineral testing kit is composed of minerals, it is preferable for the testing kit mineral to be scratched over the specimen. If this cannot be done, than the specimen has to be scratched. This should be done in an area where a scratch will not make a noticeable mark.
Hardness can be easily detected without a "kit". All one needs to know is the hardness of certain items (including the ones mentioned above) and minerals in his collection. These can be used instead of purchasing a kit, which is an inessential investment.
Specific gravity (SG) is the measurement used to determine the density of minerals. Different minerals that have the same volume have different weights. Specific Gravity is measured by the relative weight of the item to water. The specific gravity value is how many times greater its weight than the same volume of water. Water has a specific gravity of 1. A mineral with a specific gravity of 2.7 is 2.7 times heavier than water. Minerals with a specific gravity under 2 are considered light, between 2 and 4.5 average, and greater than 4.5 heavy. Most minerals with a metallic luster are heavy. The specific gravity can vary slightly within a mineral because of impurities. Geologists measure specific gravity with expensive laboratory tools, such as a hydrostatic balance. There are other methods to determine specific gravity, such as using water displacement, but this is a complicated procedure that can provide inaccurate results. Instead of testing actual specific gravity, the heft of a specimen can be noticed. It is easy to notice a very light specimen, an average specimen, and a heavy specimen (an example could be galena with a 7.5 SG compared with graphite with a 2.2 SG).
Luster describes how a mineral reflects light -- how brilliant or dull it is. The terms applied to luster are:
| Luster | Description | Examples |
| Metallic | Minerals that exhibit a metallic luster are opaque and reflective, like a metal. Metal elements, most sulfides, and some oxides belong in this category. | gold, silver, hematite, pyrite |
| Submetallic | For a mineral to fall in this category, it must be opaque to nearly opaque and reflect well. Thin splinters of submetallic minerals are translucent | Rutile, Hornblende |
| Vitreous | This luster accounts for about 65 percent of minerals. Vitreous luster has reflective properties similar to glass. Most of the silicates, carbonates, phosphates, sulfates, halides, and hydroxides have a vitreous luster. | Quartz |
| Adamantine | Transparent or translucent minerals with a very high refractive index, which means they give off a brilliance or shine. | Diamond |
| Resinous | This describe the luster of many yellow, dark orange, or brown minerals with slightly high refractive indexes -- honey like, but not necessarily the same color. | Sphalerite |
| Silky | Minerals with a silky luster are the result of the mineral having a very fine fibrous structure. The mineral displays similar optical properties to silk cloth | Serpentine |
| Pearly | A play of colors, like that of oil slick on water. Pearly luster is usually the result of many partly formed cleavage cracks parallel and below the reflecting surface of a mineral. | Labradorite |
| Greasy | If a mineral appears as if coated with grease, it is said to have a greasy luster. | Talc, Graphite |
| Waxy | Mineral appears coated with wax | Turquoise, Chrysoprase |
| Earthy, Dull | Minerals which exhibit very poor luster. Most of these minerals have a rough or porous surface | goethite |
Cleavage, Fracture, and Parting all have to do with the positioning of atoms in a mineral and how it breaks when put under stress. These three properties are together in this guide because of their similarity, but each one will be discussed separately because they are different properties.
In mineral terms, cleavage describes how a mineral breaks when subject to stress on a particular plane. If part of a crystal breaks off because off stress, and the broken piece retains a crystal shape, the mineral has cleavage. A mineral that never produces any crystallized fragments when broken off due to stress has no cleavage. Cleavage is measured by two factors: quality and number of sides that exhibit cleavage.
Minerals with
Cleavage is also measured by the number of sides exhibit it. Many minerals that exhibit cleavage only on one side, and some may exhibit different quality cleavage on different sides. We may expect to find the following criteria: One, Two, Three or All Directions.
These tell us how many directions of a mineral exhibit cleavage. Each direction means two opposite sides of a three-dimensional figure, and this is because opposite cleavage sides exhibit the same properties. If a mineral has cleavage in three directions, then every side of the mineral has cleavage, unless the mineral has more than six sides. If a mineral has more than six sides (i.e. octahedron and exhibits cleavage on all the sides, than we can call that cleavage All Directions.
Fracture is the characteristic way a mineral breaks. The difference between cleavage and fracture is that cleavage is the break of a crystal face where a new crystal face is formed where the mineral broke, whereas fracture is the "chipping" of a mineral. All minerals exhibit a fracture, even those that exhibit cleavage. If a mineral with cleavage is chipped a certain way, it will fracture rather than cleave.
There are different types of fracture a mineral can have, such as:
Tenacity is a minerals reaction to certain stress, such as crushing, bending, breaking, or tearing. There are different reactions to each type of stress. Since tenacity is composed of different reactions to different stresses, it is possible for a mineral to have more than one form of tenacity. There are different forms of tenacity, and each one must be tested separately. They are: