GO 340 Gemstones & Gemology
Emporia State University 
www.emporia.edu/earthsci/amber/go340/origins.htm
An Introduction to Gem Origin and Occurrence
Gems are found worldwide. When there is a sufficient quantity to be worked, it is termed a deposit. A deposit, as well as the location of discovery of a single find, is termed the gem occurrence. Deposits can be primary or secondary in nature. A primary deposit indicates the gem occurrence is still at the original location or at the place of formation. Primary deposits ..."have their original relationship with their host rock" (Schumann, 1997, p. 53). Crystals can be well preserved, but much of the "deaf (nongem-bearing) rock" must be removed to recover the gems (Schumann, 1997, p. 53). A secondary deposit indicates the gem occurrence has been transported to another or "second" location and deposited. During this process, harder gems become rounded, whereas softer gem material is destroyed or reduced in size. The transportation and deposition can be by rivers (fluvial), the sea (marine), along the coast (litoral), or by wind (aeolian). As the water or wind currents diminish, the heaviest gems are dropped first and become concentrated (e.g., diamond, zircon, garnet, sapphire, chrysoberyl, topaz, peridot, tourmaline).
Most gemstones are minerals and originate from melts, solution, or vapors. The atoms arrange themselves from a disordered state into an orderly crystalline state, with growth dependent upon the availability of free space. Magmatic minerals crystallize from magma, lava, or gases and are found in igneous rock. Minerals that crystallize from hydrous solutions, at or near the surface, are found in sedimentary rock. New minerals formed from a recrystallization of existing minerals beneath the surface are found in metamorphic rock.
All rocks fall into one of three classifications, igneous, sedimentary, or metamorphic. Igneous form from the cooling and crystallization of molten magma. Sedimentary rocks are composed of materials that result from mechanical and chemical weathering of preexisting rocks. Sediment is deposited in layers become rock when cemented or compacted together. Metamorphic rocks were once igneous or sedimentary and underwent a change in texture and/or mineral composition. The change is in a solid state, caused by high heat and pressure resulting from burial deep within the crust of from intrusive igneous rock. Gems can also form from solution as vein or secondary minerals.
The Earth is at least 4.5 billion years old, and probably had beginnings as a hot, fiery sphere of molten material. As the molten mass cooled, crystals formed and the Earth's surface was covered by a layer of igneous rock. Therefore, in the beginning, all rock was igneous (from Latin, meaning "fire"). Most igneous rock is made up of minerals, such as quartz, feldspar, nepheline, mica, amphibole, pyroxene, and olivine. The texture of the grains and the relative amounts of these minerals help to classify the rock. In addition to the minerals used for classification, igneous rock contains other minerals referred to as accessory minerals. These minerals can form large, perfect crystal gems, such as zircon, sapphire, and pyrope garnets. After the rock weathers, the minerals are recovered from unconsolidated sands and gravels. Igneous rocks can be classified into intrusive, extrusive, hypabssal, or pegmatite environments.
- Intrusive or plutonic igneous rock occurs when molten magma forms beneath the surface in the Earth's crust. Because of slow cooling, large crystals result in a coarse-grained texture. A common intrusive rock is granite, composed of quartz, feldspar, and mica.
- Pegmatites are igneous masses, usually related to granites, in which the crystals grew to enormous proportions. The crystals are large and the high percentage of rare and unusual elements create ideal conditions for gemstones. "Pockets" of free space for mineral growth can allow for transparent and flawless crystals. Some examples of minerals found in pegmatite pockets are: quartz, feldspar, beryl, tourmaline, topaz, spodumene (both kunite and hiddenite), chrysoberyl, and apatite.
- Hypabyssal or shallow intrusive igneous rock formed when molten material began to cool beneath the surface and was then pushed up to the surface. Larger crystals result, disseminated throughout a finer-grained groundmass, termed a porphyritic texture. This igneous rock occurs in structures such as dikes and sills, which can also act as feeders for pipelike diatremes. Olivine is weathered from dike rock in New Mexico and Arizona, whereas sapphires are mined from dike rock in Montana. Diamond, an accessory mineral which formed millions of years ago deep in the Earth's crust, traveled to the surface in igneous pipes or diatremes of kimberlite and lamproite.
- Extrusive or volcanic igneous rock is extruded onto the Earth's surface. The lava must cool quickly, forming tiny crystals that result in a fine-grained texture. Very rapid cooling, termed quench cooling, allows no time for crystals to form, and results in volcanic glass, such as obsidian, known as apache tear or snowflake obsidian when used as a gemstone. A common extrusive rock is called basalt, which is composed of feldspar and pyroxene, with or without olivine. Another source of gemstones in extrusive rock does not come from the primary mineralization but from secondary minerals which form in gas cavities or vugs. The mineral-lined vugs or vesicles in volcanic rock contain agates and geodes, turquoise and opal. [Geodes can also occur in sedimentary rocks.]
To the beginning.
In the beginning, the Earth was composed of igneous rock, but the processes of mechanical and chemical weathering broke down the rock into sediment. Mechanical weathering creates sediment that is transported via wind, water, and ice. After transportation, sediment is deposited, compacted, and cemented into solid rock. Mud turns to shale, sand into sandstone, and coarser pebbles cement together into conglomerates. Chemical weathering changes or liberates minerals in rocks. For example, igneous and metamorphic rocks have abundant feldspar, which chemically weathers into clay minerals, and quartz, which is inert to the chemical attack and freed from the parent rock. Chemical sedimentary rocks are precipitated by chemical reactions, evaporation, or organisms, resulting in gypsum, rock salt, and limestone to name a few. Rock weathers into sedimentary deposits that include eluvial, alluvial, and placer.
- Eluvial deposits occur when rocks are weathered and the resistant minerals are freed. They collect in surface or near surface concentrations. Gold is concentrated as an eluvial deposit, as is gem quality olivine or peridot in New Mexico and Arizona. Eluvial deposits are rare though, and more often the minerals freed from the parent rock and moved away from the source by the agents of erosion, wind, water, and ice.
- Alluvial or placer deposits are accumulated where the resistant minerals and rock fragments become the sand and gravel of stream beds. The grains become less angular and more rounded the farther they are transported and the minerals with higher density or specific gravity, or gem quality material, settle to the bottom of stream beds and water bodies. These minerals are chemically inert, mechanically strong, high hardness and density, such as diamond, zircon, corundum, garnet, spinel, andalusite, gold, and platinum. Placer deposits can become cemented into sedimentary rock misleading people who concluded diamond and gold formed in the sandstones or conglomerates.
To the beginning.
Metamorphic rocks form from igneous, sedimentary, or other metamorphic rocks, changing in a solid state as a reaction to high pressure and temperature and/or hot circulating fluids. The hot chemical fluids, pressure and temperature, add new minerals and textures to the existing rocks with two common methods of metamorphism, regional and contact.
- Regional metamorphism affects large areas creating gneiss, coarse-grained rock with light and dark mineral bands, and schist, foliated rock of primarily micas and quartz. The resulting metamorphic rock depends on the composition of the original or parent rock and the temperature-pressure conditions present at the time of formation. A pure limestone changes into marble, whereas an impure limestone with some aluminium present can change into a marble with mineral grains, called porphyroblasts, of corundum (aluminum oxide) or spinel (magnesium aluminum oxide), such as gem marbles found in Myanmar (formerly Burma). Schists can also host gem minerals, such as garnet, staurolite, andalusite, kyanite, chrysoberyl, and iolite (cordierite).
- When igneous rock intrudes into rock, contact metamorphism takes place, or a baking of the surrounding rock with the high temperatures. If hot fluids emanate from the magma, introducing new elements, rock has undergone "metasomatism" and possibly the formation of new minerals, such as, garnet, diopside, epidote, corundum, spinel, lazurite, and lapis lazuli.
To the beginning.
- Some minerals crystallize from solution, or water-rich hot fluids that move up through the Earth's crust or cool solutions seeping downward from the surface. The solution fills cracks in the rock and these are called vein deposits. Hot-water or hydrothermal solutions that emanate from magmas can crystallize a variety of minerals as the solution moves upward into cooler environments. Metallic ores containing gold, silver, tin, lead, zinc, and copper can originate in this way as well as the gem materials that are commonly associated with ores of the metals, such as quartz, rhodochrosite, rhodonite, fluorite, and calcite. Hydrothermal vein deposits are also found in limestones, and include emerald, albite, barite, fluorite, pyrite, sphene, anatase, and brookite.
- Gem minerals can also be a result of secondary mineralization formed by alteration or solution of preexisting primary minerals. The process takes place near the surface and brought about by chemical weathering. If solutions encounter an ore vein containing zinc or copper minerals, the minerals go into solution and leave the region. If limestone forms the vein wall, the metals in solution react with the carbonate in the limestone, turning zinc into smithsonite and copper into azurite and malachite. If silica is present, copper rich solutions can form the secondary minerals dioptase and chrysocolla. Turquoise and variscite are secondary minerals formed where the overlying rock contains phosphorous. Opal is another valuable secondary mineral formed when silicon and oxygen are available and found in veins, nodules, or as the petrified material in petrified wood.
To the beginning.
Required reading! Go to:
Where do gems form? This lecture from Jill Banfield's Gems and Gem Materials course at UC Berkeley is found at http://socrates.berkeley.edu/~eps2/wisc/Lect3.html
- Hurlbut, C. S., & Kammerling, R. C. (1991). Gemology. NY: John Wiley & Sons, Inc.
- Schumann, W. (1997). Gemstones of the world. NY: Sterling Publishing Co, Inc.
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This page originates from the Earth Science department for the use and benefit of students enrolled at Emporia State University. For more information contact the course instructor, S. W. Aber, e-mail: saber@emporia.edu Thanks for visiting! Webpage created: 1999; last update: January 14, 2008.
Copyright 1999-2008 Susan Ward Aber. All rights reserved.