Minerals, Rocks, Mining, etc.

What are minerals?
A MINERAL IS a naturally occurring, inorganic (not made by an animal) solid that possesses a definite chemical composition (or range of composition within limits) and an atomic structure; together the structure and composition give the mineral a unique set of physical properties.

Again, minerals:

Some solids are not minerals.  Glass, coal, wood, plastics, synthetic gems, pearls are examples of what are not minerals.

The Earth has about 5,000 minerals, but only about a dozen are common rock-forming minerals in the Earth's crust.

The composition and structure of a mineral are based on the elemental properties and the arrangment of atoms or molecules into a definite pattern or framework, referred to as the crystal lattice.  The crystal lattice is formed by regularly spaced chemical bonds and a repetion of a structural "theme" due to these bonds repeating.

What are gems?
Gems are typically minerals and usually are quite rare.  Gems should be durable (both hard and tenacious), and are colorful, or have physical properties, such as sparkle and fire, that make them attractive and valuable.


Matter consists of atoms. Atoms are composed of a nucleus (that contains positively charged protons and neutrons [which have no electrical charge]) surrounded by a cloud of negatively charged electrons (which have a negative charge, opposite and equal charge to a proton).  Electrons occupy shells or orbitals representing different energy levels surrounding the nucleus.

The atomic number ( the number of protons [positively charged particle] in the nucleus).  The atomic number is the basis for defining an element.  An element is a specific atom.  Since gold is a metal made up of all atoms of the same type, it is an element.  If an element occurs naturally in its pure form, it is a native element; an example would be pure carbon in diamonds.  Atoms make up all matter on Earth.  The arrangement of electrons in shells surrounding the nucleus gives the atom its electrical charge and atomic size characteristics.  The electrons are the typical cause of chemical bonding. Electrons are involved in chemical bonding which makes up the atomic structure of compounds and native elements (atoms linked together).

Valence, the number of electrons available for chemical bonding in an atom, is an important factor as to how compounds are formed.  If an element has either 2 electrons in its outer shell, such as helium (He is the only atomic element with 2 electrons in its outer shell) or 8 electrons in its outer shell, it won't readily form bonds and is called a noble element.  Most noble elements are gases.  A few metals also will not readily form chemical bonds with other element, such as gold, which for this reason will not tarnish.  Most elements do not have a full outer shell and thus want to obtain or lose electrons in order to reach a condition of a full shell (2 or 8 electrons).  When they lose or gain electrons (by either giving up or acquiring them from another element), they become electrically charged and are called ions.

Elements combine in definite proportions to form compounds. Most minerals are compounds! Based on the valence electrons of an element, the atoms of that element form different kinds of bonds.

Radioactivity is based on the instability of some isotopes. Some isotopes break down to form lighter isotopes or different elements. This is important to the study of radiometric dating.  Radioactivity is not particularly important in gemology, but some minerals may be naturally radioactive (such as some zircons, uraninite, etc.).  Some gems may be exposed to radioactivity in order to effect a change on the gem's properties (usually a color change).  Occasionally a gem can be radioactive and thus dangerous to wear.

It is best to avoid radioactive substances at all times!

Properties of Minerals

Mineral properties are a result of the type of atoms and types of chemical bonds that make up the minerals.  Minerals with the same chemical properties may behave very differently if they have different bonding.  For example, graphite (the mineral from pencil leads) and diamond are both pure carbon (a native element).  Yet because of the bonding arrangement graphite is soft and slippery to the touch, while no natural substance is harder than a diamond.  As well, diamonds can be amazingly transparent while graphite never is.

Below is a list of easily testable properties of minerals.  These physical properties are generally easily observed and are often diagnostic for an unknown sample.  Other properties not mentioned here may be extremely useful for science and technology and many mineral-like substances are synthesized by man to take advantage of a specific property.

Crystals form, color, hardness, luster, streak, cleavage, fracture, and specific gravity are the least you should know to help start identifying minerals, but some of these tests are difficult to apply to gems.  Only non-destructive tests should be used on cut gemstones.  Even diamonds can be broken if improperly handled and sometimes the best way to identify a gem is by using a specific instrument such as a conductivity or reflectivity meter or by an optical test for a cut stone.  Gem material that is uncut (gem rough) can be tested more simply and is discussed below.

For uncut gem material the following are valuable tests that can be preformed anywhere with no special equipment.

Crystal form is the external expression of the orderly internal arrangement of atoms. If a mineral can grow unrestricted, it will develop individual crystals with well-formed crystal faces. Crystal faces are smooth and shiny.  They are due to growth, not breakage.  Some minerals sold have fake crystal faces ground onto them; an example is large rose quartz wands that appear to resemble quartz crystals but are manmade.  Rose quartz only very infrequently crystallizes with crystal faces.  Crystal form may be quite diagnostic for certain minerals.  For example: diamond often forms octohedrons, 8-equilateral triangular faces (like one Egyptian pyramid on top of another); tourmaline forms elongate crystals that have striations parallel to the long direction and are often curved triangles in cross section; quartz forms 6-sided elongate crystals with striations at 90 degrees to the long axis and is usually terminated bya a pointy tip.

Other important properties of minerals that are easily observed: Color, LusterStreak, Hardness, Cleavage, Fracture, and Specific gravity. Other properties that may require additional test equipment to observe include: refractive index, fluorescence, phosphorescence, solubility, magnetism, radioactivity, piezoelectric, and "magical power " according to some folks....

A comparison of quartz and calcite:


SILICATES: (silicon-oxygen TETRAHEDRON, tetrahedrons form chains, double chains, and sheets.)

Common silicates are subdivided into groups:

BOWEN'S REACTION SERIES (p. 49, Chapter 3) - explains how the CRYSTALLIZATION process as magma cools, causes HIGH temperature minerals to form FIRST (mostly MAFIC minerals), and the LOW temperature minerals to form LAST (mostly FELSIC minerals). This causes the composition of the magma to change through time from mafic to more felsic. In a "magma chamber," a large volume of molten rock underground, this process is called MAGMATIC DIFFERENTIATION.


Aggregates of minerals.  Rocks have massive properties and do not have the internal arrangement of minerals nor the definite chemical composition.

There are 3 major categories of rocks: igneous, sedimentary, and metamorphic.

Igneous rocks (literally full of fire) are formed from melting of Earth materials.  The material originates as a melt (molten material) and forms rock by solidification due to cooling.  Melt below the surface is called magma; once it breaks the surface it is called lava. Because the Earth is not entirely heterogenous, the melt may vary in composition and it may also change its composition as it rises through the Earth's crust toward the surface by incorporating material it meets and melts along the way.  Part of the melt may crystallize below the surface and change the composition of the remaining melt (just as you can remove fat from the top of chilled soup).

Hence there are many varieties of igneous rocks.  One of great interest is Kimberlite, the rock material that holds and carries toward the surface most diamonds.

Sedimentary rocks are composed of materials that either are derived as particles from weathering and erosion of older rock or that precipitate from water as a salt.  Accumulations of sediments may hold gem material as eroded particles, for example gem gravels and placer (stream) gold deposits, or may act as a host for gem formation.  When acting as a host, the sediment is usually heated by igneous intrusions or metamorphism (a process of change) and the gems grow within the sediment or in veins in the sediment.  Examples of this would include opal and gold veins (though opal and gold veins may occur in igneous and metamorphic rocks as well).

Metamorphic rocks are usually formed by heating or pressurizing preexisting rocks to form a new and somewhat altered metamorphic rock.  Metamorphic changes extend until the rock actually melts and becomes igneous.  Examples of metamorphism include the heating of preexisting rock by intrusion of igneous rock that causes the rock to heat, bake and change its appearance.  The rock may also be metamorphosed by pressure that bends, distorts, or breaks the rock.

During metamorphism the rock is heated leading to recombination of elements and new metamorphic minerals form.  Many of these are gems, for example, emerald, ruby, sapphire, chrysoberyl, and garnets.

Any rock may hold gems and they must be studied carefully to determine whether there is a prospect of valuable deposits.

PROSPECTING is the search for valuable materials including gems, industrial Earth resources (oil, coal, bauxite, etc.), or precious metals such as gold, silver, and platinum.

Prospecting done by searching for gems that enter the rivers and streams of the world by erosion of rock is called alluvial prospecting.  You can accomplish it by using a pan and walking a stream looking for gold.  The gold is panned by swirling the flat dish-shaped pan and allowing the lighter minerals to fly out.  The gold is denser, having a specific gravity of 19.32 compared to quartz with a specific gravity of 2.65, it is more than 7 times as great and stays behind when the quartz is washed out of the pan.  Once an alluvial deposit is found, it can be traced upstream to the source rock (remember the larger heavier pieces will be upstream).

Diamonds found recently in Canada are the result of a different type of prospecting.  The diamonds were found first in glacial deposits carried down from Canada.
Diamonds have been found as far south as Iowa in glacially transported sedimentary debris.

Prospecting can be done on foot with a mule, on a boat, or from the air.  Today prospecting is even done by using satellites in space!  This is called remote sensing and relies of the spectra of  valuable minerals or associated minerals being detectable from space, or because the shape of the land or density of rock suggests valuable minerals may be found.

MINING is the extraction of valuable material from the Earth.  The valuable material occurs in ore.  Gold ore, diamond bearing kimberlite, coal, etc., are extracted by mining processes.  The processes can be simple or complex and the separation of the material can be accomplished by making use of physical or chemical properties of the material being extracted.  For example underground mining of salt is accomplished by piping in hot water and extracting the dissolved salt as brine that is then reevaporated at the surface.  Mining involves extraction, it physically damages the Earth.  The effects can be mild to severe and the ecological aspects of mining must be weighed carefully against the economic benefits.  Mining underground is dangerous work and both the fear of damage to the health of the miners and to environment add to the cost of any prospect.  Open pit mining of coal, called strip mining, has been banned in the USA because of deleterious effects which include acid runoff.  Mining of gold uses poisonous substances such a cyanide and mercury (to amalgamate the gold).  There have been accidents and there will be further accidents.   Mining must be carefully done for everyone's sake.

Return to the syllabus main site
Return to the Biological Sciences and Geology Website