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Categorizing minerals can prove to be a difficult task, as often completely different minerals can look remarkably alike. Mineralogists, however, have devised a method to help them identify minerals in the field which uses a few basic techniques geared to the physical properties of minerals. These properties include, but are not restricted to, the habit, form, cleavage, parting, fracture, hardness, specific gravity, color, luster and streak of the minerals. All of these properties are in reference to an individual mineral.  This property applies to the external shape of the mineral's growth, and it is determined by visual observation. This refers to the geometric shapes that make up a mineral crystal. Form is also visually determined. This property refers to the mineral's disposition to break along planes of weakness at the atomic scale. This weakness may be caused by an imperfect atomic bonding of some of its planes, thereby disposing the mineral to break at these weaker spots. All specimens of a specific mineral have this property. This also refers to the mineral's disposition to break along specific planes of structural weakness. This particular weakness, however, is due to the growth history of a particular specimen and does not necessarily apply to every specimen of this mineral. If the bond strength at the atomic scale is approximately the same all over the crystal, then any breaks that occur will tend to have an irregular shape. This irregular shape is called a fracture.
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This property refers to the mineral's resistance to scratching (either by a fingernail, a penny, a knife or other minerals), and is measured on a scale of 1 to 10 (called Moh's scale). 1 applies to the softest minerals and 10, to the hardest. The following minerals serve as references for the Moh's scale to help determine the approximate hardness of an unknown mineral:
This is the ratio between the weight of a mineral and the weight of water with volume equal to that of the mineral. For example, if a mineral has a specific gravity of 3.5 it means that the mineral weighs three and a half times the amount of an equal volume of water. To ensure accuracy, the temperature of the water must always be at 4 degrees Celsius, since it is at this temperature that water reaches its densest point. The following are some examples of specific gravity:
Many would think this an easy characteristic by which to identify minerals, but it is one of the most deceptive. Minerals can have a range of compositions in their make up, and this variation in composition can cause color variations as well. Although most minerals have "typical" colors, they can often be discolored due to weathering or naturally occurring inclusions (specs of impure materials). The common mineral quartz illustrates this very well. It comes in a wide range of colors, including smoky, pink, clear, opaque and purple.
This property refers to the mineral's appearance when light is reflected off its surface. Some minerals give off a very shiny appearance, while others appear very dull. A broad range of adjectives are used to describe a mineral's luster.
This property also relies on the mineral's color, but it is far more consistent in mineral specimens and does not vary within a mineral series. Streak is tested by scratching the surface of a porcelain plate with the mineral to obtain a powder residue. The color of the powder becomes an important factor in determining which mineral the mineralogist is dealing with. For example, even though hematite can be reddish and dull, some specimens appear black/silver with a metallic luster. Both types specimens, however, will yield a reddish/brown streak when scratched over a porcelain plate. Most silicates, which make up approximately 90% of minerals, yield a white or colorless streak.