Again, minerals:
The earth has about 4000 minerals, but only about a dozen are common rock-forming minerals in the Earth's crust.
Gems are typically minerals and usually are quite rare. Gems should be durable, and are colorful, or have physical properties, such as sparkle and fire, that make them attractive and valuable.
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 atomic number (that is the number of protons [positively charged particle] in the nucleus). The arrangement of electrons in shells surrounding the nucleus gives the atom its electrical charge and atomic size characteristics. Electrons are involved in chemical bonding which makes up the atomic structure of compounds (elements or atoms linked together).
Valence, number of electrons available for chemical bonding, 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 bond and is called a noble element. These are mostly 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. 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. Minerals are compounds! Based on electronic properties of atom, atoms form different kinds of bonds.
The Atomic weight of an element is based on the average weight of elements
in their relative natural abundances. e.g. C12 and C14.
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. For example, carbon
has three isotopes: C12, C13, and C14.
C14 is radioactive: it forms from incoming cosmic radiation
striking nitrogen (N14) in the atmosphere (neutron + N14
= C14 + proton ). C14 is then absorbed by living
tissue. No new C14 is absorbed once an organism dies. C14
constantly slowly breaks down through time: ( C14 = N14
+ beta- particle). Thus the amount of C14 in dead animals and
plants decreases over time and the older the animal or plant fossil the
less C14 is present. Radioactivity is not particularly
important in gemology, but some minerals may be naturally radioactive (such
as zircon, uraninite, etc.) Some gems may be exposed to radioactivity
in order to affect a change on the gem's properties (usually color change).
Occasionally a gem can be radioactive and thus dangerous to wear.
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.
Other important properties of minerals that are easily observed: Color, Luster, Streak, 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:
Common silicates are subdivided into groups:
OXIDES
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). The history of igneous rock is complex and beyond our present need to know.
Hence there are many varieties of igneous rocks. One of great interest is Kimberlite, the rock material that holds and carries towards the surface most diamonds.
Sedimentary rocks are composed of material that either are derived as particle from weathering and erosion of older rock or that precipitates 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 intrusion or metamorphism (a process of change) and the gems grow within the sediment or veins in the sediment. Examples of this would include opals 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 somehow 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 can be 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 an 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 up stream 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
it then reevaporated at the surface. Because 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 included
acid runoff. Mining of gold uses poisonous substances such a cyanide
and mercury (to amalgamate the gold). There have been accidents and
will be further accidents. Mining must be carefully done for
everyone's sake.