A mineral was defined as a naturally occurring inorganic product with a characteristic chemical composition and usually a crystal structure. Usually, rocks are made up of aggregates of one or more minerals, but they may be composed of or include either natural glasses or organic materials such as coal. They, are distinguished further by the fact that they make up a significant portion of the earth is crust. There are three major rock types, based on the method by which they formed.
Igneous Rocks
Igneous rocks (pronounced Ig-nee-us) are those that formed as the result of the cooling of molten rock masses, which are known as MAGMAS. What causes segments of the earth's crust at depth to melt has been the subject of speculation among scientists for hundred of years. Probably the most widely accepted theory for its cause today is that energy supplied by radioactivity, added to the high existing temperatures and pressures prevailing at depth, triggers the melting action. Such molten rock bodies may be very limited in extent or extend for a thousand miles or more. The core of the Sierra Nevada Mountains between California and Nevada, which has a total length of 1100 miles, was formed by the cooling of an enormous molten mass, yielding an igneous rock akin to granite. If molten rock cools and solidifies slowly at depth, it will be coarsely crystalline. Rapid cooling may produce a natural glass such as obsidian, a porous rock such as pumice, or a very minutely crystalline rock such as lava (usually known to the mineralogist as basalt). Examples of coarsely crystalline igneous rocks are granite and the slightly less coarsely grained kimberlite (or blueground), the host rock of diamonds.
To the colored-stone man, the igneous rock of greatest interest is the PEGMATITE DIKE. A dike is a lens-shaped body of igneous rock. A pegmatite dike is a type formed late in the cooling and solidifying process of a magma that has a composition similar to that of a granite. As such a mass cools at depth, many of the components start to crystallize. The more volatile constituents remain liquid for a longer period and accumulate in cracks both in the hardening mass and in the surrounding rocks. Because the mass has been molten for a long period by this stage, the temperature in the hardening mass and surrounding rocks has become very even. Further cooling will be slow. In addition, the remaining liquid is very volatile so the conditions are ideal for the growth of large crystals of fine quality. The fluidity of the volatile end products permits atoms to move about freely, so that even those present in minute quantities have an opportunity to reach other like atoms and form crystals. Thus certain very rare elements may form crystals of significant size. Although beryllium is a rare element, large crystals are sometimes formed in pegmatite dikes. Other gem minerals found in pegmatite dikes include topaz, tourmaline, kunzite, moonstone and chrysoberyl, as well as many rare gem minerals such as euclase and apatite.
Another manner in which gems may be formed in connection with igneous activity is that in which solutions emanating from the magma deposit minerals in crevices in surrounding rocks, often at considerable distances from their point of origin. The cracks filled in this manner are called VEINS. The same term is applied to similar crevices filled by deposition from circulating subsurface cool waters just below the surface. An example of the first type is amethyst or emerald and of the second, turquoise, malachite or quartz. Nodular rocks that are hollow and lined with crystals of quartz or other minerals are called GEODES (pronounced GEE-odes). The crystals are usually deposited by relatively cool solutions.
Sedimentary Rocks
Weathering agents at the earth’s surface are perpetually active in
decomposing rocks. The action of ground water, frost, chemicals plants,
streams, wind and bodies of water break up the rock. Streams, wind and
other agents carry off the fragments to be re-deposited elsewhere. These
are called SECONDARY DEPOSITS, in contrast to the igneous rocks, which
are regarded as PRIMARY DEPOSITS. Concentration of the, heavier minerals
by streams in river gravels provides the most important gem sources
among the sediments. Such deposits are termed PLACER or FLUVIAL. Almost
every gem mineral that is reasonably durable is found in alluvial
deposits. Thus, with the exception of opal, turquoise, malachite,
kunzite and a few other gemstones, most gem minerals are produced in
important quantities from gravels.
The important sedimentary rocks in order of decreasing grain size are
conglomerate sandstone, shale and limestone. Since conglomerate
represents gravel that has been consolidated into rock, some gem
production comes from conglomerates. With this minor exception,
sedimentary rocks of themselves are of minor importance to the
gemologist. However, many vein-type deposits are found in sediments;
e.g., opal and turquoise.
Metamorphic Rocks
The derivation of the word METAMORPHIC (pronounced met-uh-MOR-fik) explains its meaning. It is from the Greek words META (meaning CHANGE) and MORPH (meaning FROM). Metamorphic rocks are those (that have been changed from their previous form by agencies such as heat, pressure, and or action of near by molten masses. The major type, REGIONAL METAMORPHISM, is of limited interest to the gemologist, for little of gem quality results. In this type, the heat and pressure generated by tens of thousands of feet of overlying rocks, or that which accompanies the following of rocks during mountain-building activities within the crust, cause drastic changes. Sandstone becomes quartzite, shale becomes slate, limestone becomes marble and platy rocks such as schist’s form. During such processes crystals often grow in solid rock! Although garnets form under these conditions, they are seldom of gem quality.
The type of metamorphism of prime importance to the gemologist is that called CONTACT METAMOPHISM. In this type, existing rocks are "cooked" by the intrusion of a nearby mass of molten rock. The combination of them resulting increase in temperature and the solutions traveling out from the magma often cause the original rock to re-crystallize. In the process, impurities from the pre-existing rock often become concentrated and form new minerals. When impure limestones are crystallized under these conditions, corundum (ruby and sapphire) may form, as well as spinel and other gem minerals. Mongol, Burma, by far the most important single colored stone locality in the world, is such a deposit.
GENERAL COMMENTS
The background of geology and mineralogy relating to the formation of
gemstones, contained in this assignment suffices for the student
concerned primarily with gem identification, appraisal and sales.
However these are subjects that many people find fascinating. After you
have completed this course, you may want to explore the field further.
If so, your instructor will be happy to recommend a number of books that
are likely to be available in your local library.
