| GEM FORMATION Before venturing into a summary of the ways in which gems are formed, it would be good background to review where those processes are occurring. A crude geological model of our Earth can be made by using an apple. The Earth's crust or top layer (ranging from 3 - 25 miles deep, only 1% of the Earth's volume) is represented by the thin skin, its mantle accounting for over 80% of the volume is represented by the flesh (over 1800 miles thick), and the core is represented by, what else?, the core.
Approximate size relationship of Earth's three layers : crust (peel), mantle (flesh), core (seeds and surrounding core) The core has a solid inner, and liquid outer, portion, and is the least known and studied part of the Earth. Most of the mantle consists of melted rock (magma) and giant segments of the Earth's crust along with the solid upper mantle, called "tectonic plates", float on this fluid rock and move slowly over it. The movement normally occurs at about the same rate your fingernails grow, but can be substantially faster when one plate suddenly slips in relation to another. The various ways in which these plates encounter each other, are responsible for many of the momentous events that shape our Earth, such as volcanism, earthquakes, and periods of mountain building.
Crustal Plates Almost all gems of mineral origin form in the Earth's crust, with the notable exceptions of peridot and diamond, which form in the mantle, and all of them are mined in or on the Earth's crust. This gemiferous crust is made up of three types of rocks: igneous, sedimentary and metamorphic, which differ in their origin and characteristics. Igneous rocks are those which solidify from a molten state, sedimentary rocks form due to consolidation of layered sediments , evaporates, or precipitates, and metamorphic rocks result when great temperature and pressure change the crystal structure of either igneous, sedimentary, or other metamorphic rocks. (Recall from Lesson One, that minerals are composed of a single substance, like quartz, and rocks are made of more than one mineral, like granite, for example, which consists of quartz, mica and feldspar.) 3 TYPES OF ROCKS
Igneous : basalt from lava flow
Hawaii, Sedimentary : sandstone hills
Utah, Metamorphic : rocks in Switzerland The Rock Cycle :
The rocks that we find on, and under, the Earth's surface are involved in an age-old, and continuing, recycling program of Nature called the "Rock Cycle".
The Rock Cycle Let's start with the red area at the bottom of the diagram: magma, molten rock. When magma rises through cracks and cools slowly underground, it forms igneous rocks composed of minerals with fairly large crystal sizes, these are known as intrusive igneous rocks. When the magma erupts onto the surface, as through a volcano, it is termed lava, and depending on the rate of cooling, the extrusive igneous rocks which form have medium to very small mineral crystals. (Review the concept of temperature and crystal size from Lesson Three, if necessary). Some lava cools so rapidly that it forms an amorphous material without a crystalline structure. Granite and basalt are examples of larger and smaller grained igneous rocks, respectively, and obsidian (volcanic glass) is amorphous.
Granite : note coarse texture Once the igneous rock is on the surface, the forces of erosion and weathering produce smaller particles which accumulate on the surface, and/or are moved by wind and water. As time proceeds, layers of these sediments build up (on land or under water). The pressure from upper layers causes compaction in the lower layers along with various chemical and physical changes (lithification), which lead to the creation of sedimentary rock.
Evaporation is an alternate factor which also produces sedimentary rocks, as when dripping mineral-laden waters leave behind stalactites or stalagmites. Likewise, surface or subterranean waters carrying dissolved minerals may evaporate or precipitate those minerals within the cracks in other rocks, or between rock layers. Sandstone and limestone are familiar sedimentary rocks formed by lithification. Opal and turquoise illustrate the evaporative mode of formation.
The presence of intrusive magma in a local region (contact metamorphism), or of tectonic plate interactions on a larger scale (regional metamorphism) puts the igneous and sedimentary rocks and minerals under heat and/or pressure which may cause changes in their chemistry and crystal structure. The result is the creation of metamorphic rocks. Thus is limestone turned into marble, sandstone into quartzite, and serpentine into nephrite jade.
As with most cycles in Nature there are sub-cycles and cross interactions. So, for example, sedimentary rocks which are subducted through tectonic action may melt and form magma which produces igneous rocks. Or metamorphic rocks, which have been uplifted and exposed at the surface, will erode to form sedimentary deposits.
How and Where Gems Form :
A specific, and unlikely, combination of five factors: temperature, pressure, space, chemical elements, and time, are required for the formation of each kind of gem. This is why gems are, in general, rare but some are rarer than others. Silicon and oxygen are the two most abundant elements of the Earth's crust, and the conditions for the formation of quartz (SiO2) are relatively common, so it is understandable that quartz is found widely. Axinite on the other hand, which is also a silicate gem, requires (in addition to silicon and oxygen) calcium, iron, magnesium, boron and aluminum for its formation, and is much rarer.
Percentage of Earth's Crust Composed of Various Elements :
Oxygen : 46.60%
Silicon : 27.72%
Aluminum : 8.13%
Iron : 5.00%
Calcium : 3.63%
Magnesium : 2.09%
Boron : 0.0010%
Beryllium : 0.00026%
Gems, in Nature form: 1. from solutions by precipitation, 2. from melts by crystallization, or 3. from vapors by condensation. (when Man synthesizes gems, these are also the three possible modes of production.)
Solution/Precipitation Gem Formation :
Both near-surface, cooler waters, and warmer waters from lower depths in the Earth, can dissolve certain minerals from rocks or sediments, and carry, mix, and concentrate them until conditions change, ultimately precipitating them as solids (crystals or amorphous materials).
Near surface environments: Near surface waters, like rainwater, move down or up, through soil or rock, as the local cycles of precipitation and evaporation dictate. Such water has carbon dioxide from the air dissolved in it, which creates a weak acid solution (carbonic acid) in which many minerals are soluble. If the environment contains sandy soils or sandstone rock, then silica will be dissolved, and certain silicate gems such as aggregate quartzes, like agates, or amorphous opals may form as the water evaporates.
Commonly, layered or banded patterns are seen in the agates indicating cycles of formation from waters of slightly different chemistries. The botryoidal habit is also frequently seen in gems formed under near surface conditions. Likewise ocean water or other brines can evaporate as climate changes leaving behind dissolved minerals, like halite (the mineral name for sodium chloride, table salt). Other waters containing sulfur may evaporate, and leave behind sulfate minerals like gypsum.
If the rocks or soils contain aluminum and copper in addition to silica, then copper containing minerals like azurite, malachite and turquoise may form. NEAR SURFACE SILICATE GEMS
Agate Cabochon showing layered structure
Precious Opal in rock seam
Common Opal nodule
Amethyst stalactite : note layered structure of both aggregate and single crystals MINERALS FROM BINARY EVAPORATES
"cranberry halite" from Nevada
Green Halite from Australia (color is due to pigments from crustaceans and microorganisms that lived in the salty water)
Gypsum "roses"
Turquoise bearing rock, from Nevada
Rare occurence of single Turquoise crystals from Virginia 50x
Malachite
Turquoise and Chrysocolla veins in an Arizona rock
Slice of a Malachite and Chrysocolla Stalactite Think about where you imagine miners finding agates, opals, and copper minerals you probably already know that the best deposits occur in rocky, sandy areas with an arid or semiarid climate. (Most of the world's precious opal, for example, comes from the Australian desert, and the Western USA and Mexico, are well known sites of turquoise and agate deposits).
Petrifaction
:
Slice of Fossil Palmwood
Cabochon of Fossil Palmwood from Texas
Fragments of Fossilized Dinosaur Eggshell
Pendant with red Spinel
Fire Agate and Fossilized Dinosaur Eggshell, Fossil wood slices from Oregon
Opalized
clam Fossil (opal solution filled the cavity of the clam shell and
solidified before the shell decayed. Remnants of the fossil shell
were then cut and polished away, revealing a perfect "cast"
of the original shape
Deeper Environments :
Gold veins in quartz
Native copper veins in Arizona rock
Hydrothermal Fluorite crystals
Dendritic silver in quartz
Natural Hydrothermal Emerald crystals in matrix
Geodes :
Small quartz Geodes
Huge Amethyst "cathedrals"
The outside and inside of a rare Azurite Geode from Arizona Melt/Crystallization Formation :
As magma cools, various minerals form, depending on the temperature and pressure at a particular location and time. As each type of mineral forms it reduces concentration of, or removes, some of the elements required for its formation. Thus as the mix of elements present, and the physical conditions change, so do the minerals which form.
Intrusive
:
Extrusive
:
Topaz crystal from China
Spessartite Garnet crystals in Microcline matrix from China
A huge, non-gem quality Garnet crystal in host rock Gems formed in the mantle :
Peridot crystals form in magma from the upper mantle (20 to 55 miles deep), and are brought to the surface by tectonic or volcanic activity where we find them in extrusive igneous rocks. Diamonds were formed many millions of years ago, deeper in the mantle (around 100 - 150 miles below the surface), at extreme temperatures and pressures. These diamond forming magmas would later erupt (still holding the diamonds) to form rocks called kimberlites and lamproites.
Diagram courtesy of The International Gem Society and Don Clark The scenario goes something like this: 1. magma, containing diamond crystals, suddenly and explosively finds a path to the surface. 2. As the lava (orange) rises, some of it cools and solidifies underground forming a carrot shaped formation of kimberlite rock, in which the diamond crystals are "frozen". 3. & 4. The volcanic cone has eroded away leaving diamonds at the surface, and underground in the kimberlite (or lamproite) "pipe" (gray).
Diamond crystal in Kimberlite rock from Russia
Peridot crystals in basalt from Arizona Pegmatites
:
Aquamarine crystals from N. Pakistan pegmatite
Pink tourmaline rough from a pegmatite formation in the Stewart Mine in California
Rose quartz from a Brazilian pegmatite mine Vapor/Condensation Formation :
It might be a little difficult to imagine vapors condensing to form crystals, as it seems somewhat foreign to every day experience. And it's true that at normal atmospheric pressures, and common ambient temperatures, this doesn't happen very often. But there's one good example that we can all look to: frost which forms on our windowpanes or car windshields, is, in fact, precisely a situation of a vapor (water vapor) condensing to a solid crystal (ice). The next time you get a chance use your loupe to example that frost: beautiful! Given the extreme environments created by some geological events, such as an eruption of magma, conditions can be ideal for such condensation processes, and they are relatively common events.
Vugs :
Gems cans crystallize from these vapors which are trapped and concentrated inside the openings. Often they form singly, without attachment to the surrounding surface. When we see a doubly terminated crystal, or one that is perfectly formed with no attachment point (called a "floater"), often it has formed in just a such a gas pocket. One of the most famous deposits of these doubly terminated crystals is the rock crystal quartzes formed in Herkimer, NY, and known as "Herkimer Diamonds".
Other pockets which do not produce crystals from gases, may later be invaded by surface water, or hydrothermal fluids, and become filled or lined with small or large crystals forming geodes or other similar formations.
Spessartite garnet "floater" crystal from Namibia
Doubly terminated rock Crystal quartz ("Herkimer Diamond") from New York
Igneous vug lined with hydrothermally derived Quartz crystals
Vug from Germany, containing stalagtites covered with tiny Quartz crystals Crystal growth from solutions or vapors can also exploit fortuitous openings as seen below. This ancient clam's death, and subsequent fossilization, created a space in the surrounding rock which later became home to the beautifully formed calcite crystals in this prize specimen.
Fossil clam shell with calcite crystals, from Okeechobee County, Florida Changes after Formation (Metamorphosis) :
As mentioned previously, heat and pressure from contact or regional sources can cause one mineral to metamorphose into another. This frequently occurs with gem minerals. Marble and lapis lazuli are gem rocks formed metamorphically, and rubies, spinels and garnets are gem minerals are often crystallized within rocks that are undergoing metamorphic changes, due to heat and pressure.
Ruby in metamorphic Quartzite rock
Lapis Lazuli mosaic bust
Polished Marble "egg" from metamorphosed limestone from Arizona Just a Few Words About Organics :
Thus far only those originally organic gems that have become fully mineralized through petrifaction, have been referred to in this lesson. Another lesson of equal length could be written about the precise mechanisms by which the various true organic gems come to be. Such coverage is beyond the scope of this course, however; so the following inadequate summary must suffice. (Review the definition of mineral vs organic gems from Lesson One, if necessary).
Organic gems are derived from either :
1. The secretory activities of organisms that were living (or recently dead) at the time of the harvest of the gem material.
Examples would include pearl, bone, horn, ivory, tortoiseshell and coral. Such secretions might be entirely mineral, as with the calcareous corals, for example, or entirely organic as with tortoise shell, or a combination of both mineral and organic components as with pearl, proteinaceous corals, and ivories.
Warthog Tusk (ivory)
Victorian Ivory brooch
1930's natural Tortoise shell brooch
Raw Black Coral branches (proteinaceous type)
Polished Black Coral branch pin with Tahitian Pearl 2. The secretions and structures of organisms long dead which have, over time, undergone geological and/or chemical changes from their original state.
Examples include amber, copal, jet, and bog oak. Chemical oxidation or reduction, compression, dehydration, or polymerization, have changed their original properties, but these materials still consist, at least partially, of organic molecules.
Circa 1880 "bog oak" brooch (a semi-fossilized wood from ancient peat bogs, similar to, and used as a simulant for, jet)
Amber rough : "Burmite" very rare Burmese variety, noted for its cherry red color, typical Baltic Amber Where Gems are Found and How they are Mined :
An important distinction must be made between the place where a gem forms, and where it is mined or collected (these two, most often, are not the same). The places where we mine or collect gems are known as gem deposits, and these are classified as either primary or secondary.
Primary Deposits :
A primary deposit is one in which the sought-after material is still held within the original site of its formation. These "lode" deposits are often located deep underground, and encased in solid rock (pegmatites, veins, pipes, etc.) They are, in general, likely to require substantial monetary outlay in personnel and equipment for recovery.
Although metal ores (does the famous Comstock Lode come to mind?), are frequently mined from primary deposits, it is rarer with gemstones. In certain locations, though, diamonds, and colored gemstones can be profitably mined from such sites. Techniques involve either tunneling deep into the Earth, or using open pit technology necessitating removal of massive amounts of "overburden" to get to the deeper gem bearing layer.
A consideration which is important in this type of gem mining is that the typical blasting and crushing done with metal ore materials can harm fragile gem crystals, so that much of the work must be done by slower and more labor intensive hand work. Sapphires in the US, have been "hard-rock" mined, off and on (depending on economic factors) in Montana, primarily at Yogo Gulch. The deposit there consists of sapphire cyrstals in a lamproite pegmatite dike. Although they are some of the highest quality blue sapphires in the world, lacking color zoning, and possessing an "out of the ground" cornflower blue color that requires no heating, the extreme prices necessary to repay their mining costs, limit their marketability.
Yogo Sapphires : before and after faceting. Yogos are among the most beautiful and expensive in the world Secondary Deposits :
Although a primary deposit may have been formed deep in the Earth, uplift, crust folding, or other geologic events can bring it to, or very near, the surface. All exposed surface features are subject to erosion and weathering, and this is true of gem deposits as well. The agents of erosion will then act to release the gems from their primary sites, and they collect in new secondary deposits. Secondary deposits are classed as either eluvial or alluvial depending on their relationship to the original source.
Eluvial :
When the softer more easily weathering primary structures simply release the harder and tougher gem materials, and the gems can be found at the site of decomposition, the deposit is eluvial. The gems can then be located within the debris, and generally it will be a relatively inexpensive process to gather and remove them. Additionally, the host rocks, which may contain valuable primary deposits can usually be easily located for other types of mining.
Eluvial gem rough, although often large in size, tends to be internally fractured, and quite angular and irregular in shape, which can limit its potential as faceting material.
The world's largest peridot mine, located on the San Carlos Apache Reservation in Arizona, is an eluvial deposit where the peridot is weathering out of the volcanic basalt primary source.
Arizona peridot eluvial rough, note fractured area and angularity of pieces Alluvial :
More common, and in most cases, more desirable, are alluvial (also known as "placer") deposits. The gems in these have been transported from the original site of their release, usually by water, but also possibly by wind or ice. As most gems are both denser and harder than most rocks, they accumulate on the bottom along with gravel, sand and mud, in eddies and pools in streams, rivers and along coastlines. (They can also be found in sites that had flowing water in the past, but have long since dried up).
The abrasive and frictional forces that occur as the gems are moved downstream cause the weakest parts to break off, and the edges to become more smooth and rounded. Alluvial rough, though usually relatively small, is often of high clarity, and superior faceting quality. The longer the distance the rough has traveled, the smaller and more rounded it becomes. Alluvial diamonds are an exception to this rule, in that they are harder than the surrounding rocks, and unless fractured or cleaved retain their original structure and size.
Australian spinel rough from an Alluvial deposit By far, the greatest amount of economically profitable gem mining is done by exploiting secondary deposits. Techniques range from simple one person panning and screening operations, to large scale dredging and hydraulic washing/sorting by big companies.
Alluvial Diamond miners in Sierra Leone
Within alluvial "gem gravels" several different types of gems may be found together, reflecting the various eroding primary sites within the local drainage area. Tracking back to the primary source of a particular gem ("Mother Lode") is usually very difficult, if not impossible. |
