Gem Origin and Occurrence
http://www.emporia.edu/earthsci/amber/es567/origins.htm
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.
Rocks
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.
Igneous
The Earth is at least 4.5 billion years old, beginning 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 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, called 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 pegmatites 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 finergrained 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.]
Sedimentary
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.
Metamorphic
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.
Gems from Solutions
 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.
References:
 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.