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Chapter 8 - Gems From Sedimentary Deposits

Gems in Sedimentary Deposits

Whatever precious gems and metals were mixed in the weathered rock are also released and moved along by erosion. In order for gems to survive, they must be durable (meaning with "good tenacity"). Durable gems resist the wear-and-tear of the erosion transport process and are concentrated with the denser sedimentary particles. Much of our gold, diamonds, and other precious substances are mined from sedimentary deposits, mostly ancient alluvial deposits. Whether or not sediments or sedimentary rocks contains gems or precious metals depends on the source regions where the sediments originally were eroded. Sedimentary processes can refine sediments and concentrate gems—as illustrated by "peridote" (olivine crystals) on Hawaii's Green Sand Beach (Figure 8-1).

How rock fragments are sorted by size and density is important in relation to gem and precious metals found in sediments or sedimentary rocks. Would a gold pebble (nugget) of ½ inch in diameter be deposited in a quartz pebble conglomerate with ½ inch pebbles? No. The size of the gold particle would likely be smaller to be deposited with the quartz because gold has a density of 19.3 gm/ml while quartz has a density of about 2.65 gm/ml. Gold is about 7 times denser. There must be a hydraulic equivalency (water equivalency) of particles for them to be deposited together. In other words, the gold pebbles or sand grains would be about 7 times smaller than the quartz for them to be deposited together. Though there are reasons why this may not be entirely accurate, such as particle shape. This concept of "hydraulic equivalency" is important relating to the occurrence of precious gems (such as diamonds, sapphires, etc.) and precious metals (gold and platinum) in sedimentary deposits.
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Green Sand Beach, Hawaii
Fig. 8-1. Hawaii's famous Green Sand Beach consists of olivine sand - a concentration of tiny crystals of peridote (unfortunately too small for use as gemstones!). The olivine crystals erode from volcanic rocks exposed near the beach.
Winnowing of Minerals by Fluid Flow

Hydraulic equivalency is dependent on particle size, shape, and density of target gem and precious metals in a sedimentary deposits (including unconsolidated sediments or consolidated sedimentary rocks). Size sorting, shape sorting, and sorting due to density all are at work when a particle is either entrained (picked up) or deposited (dropped) from a fluid. Mechanical concentration leads to a separation of lighter minerals from heavier minerals and allows particles of approximately the same density and shape to be deposited together. Part of mechanical concentration is winnowing. Winnowing involves lighter materials being drawn off by a fluid (flowing water or wind). Illustrated, winnowing is used to remove the outer husk from wheat. The wheat is tossed into the air and the lighter husks (chaff) are separated from the heavier wheat berries. Such sorting processes happening in a stream tend to concentrate gold, diamonds, etc. into a small volume of rock that makes their mining economically possible. Figure 8-2 shows how wave swash on a California beach winnow minerals in sand: green (epidote), magnetite (black), and quartz (white).

Placer Deposits

Many important gems and precious metals are found in accumulations of sand and gravel, typically alluvial deposits (both modern and ancient). Alluvium is material moved and deposited by streams and rivers, ones that have retrievable gems or precious metals are called placers. "Placer" is appropriately named, in Spanish it means "to please," it also means "alluvial sand." Placer deposits are of particular interest to gem and precious metal hunters. This is because the gem gravel contained in placer deposits can have the most precious of stones, diamonds, rubies, sapphires, jade, gold nuggets, etc. Both gold and diamonds are durable and will last for long times without being destroyed by abrasion or weathering processes.

A placer is a deposit of valuable material that has been concentrated in one place by natural actions such as a flowing stream or wave action that deposits the heavier material and winnows the lighter material. Dense gems, metal, and coarser-grained detrital material make up the deposits. One ubiquitous (always present), or at least nearly so, mineral in these sands is magnetite. The black magnetite makes “black sands” (Figure 8-2 and 8-3). Like quartz, magnetite is durable and relatively stable in the surface environment. Whereas quartz has a density of 2.65 g/cm3, magnetite has a higher density of 5.175 g/cm3. As a result, materials like gold, diamonds, etc. that have higher densities than quartz may also be concentrated in blacks sands. Placers may also occur on a beach or offshore bar deposits, or due to winnowing by wind as well. However, the most common placers are alluvial placers.

Modern alluvial deposits (those sediments deposited from rivers and streams and still un cemented) are one of the best places to start a search for gems and precious metals, because their drainage basins (see Figure 8-4) may encompass many square kilometers of area and the sediments derived from weathering of the landscape of the region are then concentrated into a few or single river channels. Rivers are thus followed by prospectors (those seeking valuable earth materials) from downstream upstream towards the river’s head where it begins. Any branches (tributaries) leading into the main stream must also be investigated. The trail of gems or precious metals is followed back to its source. Rivers are both an easy place to travel through rough terrain and pretty easy pickings for gem materials. Gem and metal-bearing placer deposits can be scattered throughout a river floodplain, both on the surface or buried beneath younger sediments. Alluvial placers are most common where a river makes a turn or curve, called a meander. Figure 8-5 shows meanders, and because the river turns, water is slowed causing deposition on a point bar. The point bar is on the inside curve of a stream meander. There is more likely to be a concentration here than in a straight stretch of a river channel. Point bars, and for that matter any bars in a river channel, tend to form layers (beds) of gravel. Coarser bedded deposits of sediment, such as pebbles in gravel, are good places to look for valuable materials, such as gemstones, though gold particles may be of any size.

Ancient sedimentary rocks may have a similar origin to present day sedimentary deposits, but they are typically cemented together by mineral precipitates.Over geological time, ancient alluvial deposits will have solidified into hard sedimentary rocks. In addition, climatic conditions have changed in a region so that the ancient deposits are nowhere near a modern stream. To understand these deposits and locate them we need to understand how the detrital (made of particles) sedimentary rocks are formed, identified, and classified. Detrital rocks are formed when accumulated particles (detritus) varying in size from clay and silt to large boulders are deposited. The particles (including gems and precious metals) may start their “journey” in a landslide where coarse angular rocks cascade down the side of a mountain. The rock and mineral particles may be transported no further and when cemented form a detrital rock characteristic of this bottom of a cliff or slope environment. The word "colluvium" is sometimes used to describe this detritus that is an accumulation of rock debris on a slope or the base of a cliff. "Colluvium" can decompose or disintegrate as materials weather, concentrating the more durable materials into deposits called "eluvium". Once these materials move under the influence of stream erosion and deposition they are called "alluvium". If the particles have been moved primarily by gravity, they will be angular and a rock called breccia (made of angular broken pieces of all sizes) can form. But the particles may also be picked up and rolled down the hill by running water. Fast moving streams start in mountainous areas and during floods may move particles of very large size.

Placer deposits can also experience deep burial and metamorphism long after they were initially deposited as sediments. However, because they have similar origins, their extent and the way they formed can be modeled based on recent deposits. Though cemented and metamorphosed rocks are harder to work, they may be very significant, such as gold in quartz conglomerate of the Witwatersrand of South Africa that may account for 40% of all gold ever mined.

Though Brazil is no longer a big producer, it was a major source of alluvial diamonds from about 1730 till around 1870. Diamonds from ancient stream deposits account for much of Brazil’s production. The diamonds came from older conglomerate (pebbly rocks) that were weathered into present day streams.

Separating Gems from Sediments by Panning and Sluicing

Deposits of particles from a fluid such as wind or water that are likely to hold gold, platinum, or precious gems are mostly tested by panning or sluicing rock material.

Gold panning involves allowing water to move particles, entrain them in the water, and then settle and separate them (Figure 8-6). Panning uses settling in water. It tends to allow the densest materials to settle most rapidly in the pan’s center. Size and shape would have some effect, but usually are outweighed by density. Sluicing uses a box like channel with ridges, called riffles (Figure 8-7), on the bottom to trap denser materials, such as gold, in the troughs (bottoms) of the riffles, while less dense particles of material such as quartz are washed over the riffles and out of the sluice box by running water. Figure 8-8 illustrates an example of a small placer mining operation.

In nature, stream bottoms (also called stream beds) have natural riffles and potholes that tend to concentrate denser materials. Prospectors know to try these spots when looking for precious materials. One wise prospector explained that every crack and crevasse in a dry stream bed needs to be investigated. Careful investigation may lead to success. Devices such a sluices (see below) and large dredges (boats that remove and process bottom sediments) are used to concentrate the precious contents. Dredging pulls loose sedimentary material off the bottom of the river channel.

Sedimentary rocks bearing precious metals and gems must be crushed into small fragments. In large mining operations, precious metal ores are processed using often dangerous chemicals (like cyanide) to leach out the metals from the ore. With gems, the crushed ore is sieved into suitable size fractions before gems are selected through a variety of means (depending on the gem).
ano nuevo magnetite sand
Fig. 8-2. "Black sand" winnowed from beach sand and gravel deposits. Magnetite (black) brownish green (olivine and garnet), and quartz (white) naturally sort by density.
Conglomerate rich in magnetite enough to hold a magnet
Fig. 8-3. A conglomerate (benitoite gem ore) with enough black sand that a magnet will stick to it.
drainages
Fig. 8-4. Rivers start in 1st order (1) streams that flow into successively larger stream. An area of many thousand square miles may drain into the largest (5th order streams).
Floodplain placer deposits
Fig. 8-5. Modern and ancient placer deposits on a stream floodplain. Placers are gravel bars deposited by strong currents where fine-grained materials are carried away.
Panning for gold and gems
Fig. 8-6. Panning for gold.
Riffles in a sluice box
Fig. 8-7. Inside of a sluice. Riffles are ridges that catch the denser particles, less dense particles are washed away.
Placer mining operation of an Idaho gold mine
Fig. 8-8. Historic placer gold mining in Idaho.
 

Non-Clastic Sediments and Non-Clastic Sedimentary Rocks

Non-clastic sediments include materials that are not formed by the weathering and erosion of rock, but include sediments formed from mineral precipitation directly from water or formed by accumulation of plant material and the skeletal remains of plankton, shells, bone, or other biological materials (Figure 8-9).

Mineral Precipitates From Seawater and Briny Freshwater

Mineral salts precipitate directly from seawater or briny lake water in desert environments. Normal seawater is about 0.35% dissolved matter and 99.65% water. Of the dissolved component, more than 99% consists of 6 major ions (chlorine, sodium, sulfate, sodium, magnesium, calcium, and potassium). The remaining 0.6% includes all other known elements, mostly in concentrations in of parts-per-thousand to parts-per-trillion ranges (Figure 8-10). Other "important" rock-forming elements and ions related to chemically-precipitated minerals include ions of carbonate (-HCO3), silica (--SiO4), ferric and ferrous iron (Fe++ and Fe+++), and phosphate (-PO5). The reactions that drive precipitation of salts and other minerals from seawater and freshwater are often complex. Variables including temperature, pressure, factors of eH (oxidation-reduction potential) and pH (acidity vs. basicity) control whether elements will dissolve or precipitate from water. In many surface and subsurface environments, biological activity, mostly on on a microscopic level, is an important factor.

Cold seawater holds dissolved carbonate ion (--CO3) and will release it when it warms up (combining with calcium ion to form calcite or aragonite). Most of this reaction takes place associated with the respiration and excretion processes within plankton, algae, and invertebrates living on or near the seabed in tropical or subtropical settings. In contrast, silica and most other ions are more soluble in warm water than in cold water.

As seawater is concentrated by evaporation, the first materials to precipitate will be calcium carbonate (CaCO3) as limestone (including calcite and aragonite), followed by gypsum rock (including minerals gypsum [CaSO4·2H2O] and anhydrite [CaSO4]), then rock salt (NaCl, the mineral halite), then potash (KCl; the mineral sylvite) and all other salts last. In terms of total volume of material precipitated from seawater, limestone accounts for the vast majority of all non-clastic sedimentary deposits worldwide. In warm ocean environments calcium carbonate will accumulate, whereas other salts will only precipitate under restricted settings where greater evaporative concentration can take place.

Salts and Evaporite Minerals

Many interesting and attractive minerals form from precipitation out of water in a variety of geologic settings including on surface and in open pockets underground, desert playas (dry lake beds), and marine environments. Many of these minerals are salts (precipitated out of salty water). In some cases, the rocks formed from the "drying out" (evaporation) of salty water. Rocks salt (mineral halite [NaCl]) is commercially mined from ancient deposits and manufacture by concentrating salty water in desert and coastal evaporation ponds (Figures 8-11 and 8-12).

Sulfate Salts

Rock gypsum (includes gypsum [CaSO4·2H2O] and anhydrite [CaSO4] (molecule lacks water). Gypsum is extensively mined and used primarily for construction plaster. It has two crystal forms including satin spar (or alabaster) (Figures 8-13) and a clear variety, selenite (Figure 8-14). Barite [BaSO4] can form in showy "rose-like" crystal masses (Figure 8-15). Epsomite [MgSO4·7H2O] is the medicinal ingredient in Epson Salt. Other important evaporite minerals include borates (such as the mineral ulexite), nitrates (used in explosives and fertilizers), halides (salts with chlorine, fluorine, or bromine), and others. Though these rocks and their mineral constituents are attractive, no "true precious gems" form in this way although many semiprecious stones come from rocks of non-clastic origin. Fluorite [CaF2] with its Mohs harness of 4 is sometimes used in jewelry settings (Figure 8-16).

Carbonate Minerals and Rocks

Carbonates are the most common and abundant forms of materials of non-clastic origin. Carbonate minerals can form beautiful "showy" crystal masses, but are generally to soft for used in durable jewelry (Figures 8-17 and 8-24). Calcite [CaCO3] and dolomite [(Ca,Mg)(CO3)2] are the dominant minerals in the sedimentary rocks limestone and dolostone (respectively). Aragonite is calcium carbonate with a different crystalline structure than calcite. Carbonates are produced mostly by biological activity in seawater environments—producing coral, shell, and algal reefs (referred to as "skeletal accumulations") and lime mud deposits, but can also be precipitated directly from seawater, groundwater, and fresh surface water if conditions are right. Fine-grained sediment composed of carbonate materials is called lime mud. Once consolidated into stone, lime mud and limey skeletal materials become limestone. Limestone also forms in freshwater settings, such as in streams or "speleothems" found in caverns (stalactites, stalagmites, and other features). Freshwater limestone is called travertine. Dolomite forms from the gradual replacement of calcite exposed to magnesium-rich briny waters. Through geologic time, many ancient limestones formations have been converted to dolostone.

Crystal masses of calcite, dolomite, and other carbonate minerals can be quite showy and are common crystals displayed in museum collections. They also commonly host beautiful fossils that can be used as jewelry or display. Pearls are composed of aragonite. Corals, shells, and other carbonate skeletal remains are composed of calcite and aragonite. Aragonite is not-quite-as-stable mineralogically and re crystallizes to calcite or dolomite in the process of lithification over time.

Other "showy" carbonate minerals produce beautiful crystal masses. Rhrodochrocite (MnCO3) has a ruby-red color. Celestite (or celestine) and strontianite (both SrCO3 but with different crystal structures) have typically pale blue crystals. Smithsonite (ZnCO3) can be greenish to turquoise blue, and azurite (Cu3)(CO3)2(OH)2 is dark blue. Magnesite (MgCO3) is typically bright white, and siderite (FeCO3) is typically pale brown. There are many other carbonate minerals, but these are ones commonly used for cutting, shaping, and polishing into cabochon-style jewelry. Carbonates are generally too soft to be faceted for use as ornamental jewelry. Large blocks of carbonate rocks are commonly used for sculptures or artistic building stone. A variety called lithographic limestone is used in printing.

Silica

Silica is a colorless, un reactive compound composed of silicon dioxide. Because silicon and oxygen are the two most abundant element on Earth, silica is abundant practically everywhere. It pure crystal form it is the mineral quartz (both hard and abundant). Silica is highly insoluble in normal surface conditions, however, traces of silica are present in seawater and freshwater, and its solubility is enhanced with with acidic conditions. Silica (as varieties of microcrystalline quartz) is a common cement in sedimentary rocks, particularly sandstone. Quartz occurs in a variety of forms in igneous, sedimentary, and metamorphic rocks.

Silica forms both crystalline quartz and several amorphous and microcrystalline varieties. Pure crystalline quartz is offend called "rock crystal" and is used for many ornamental and commercial products and is possibly the most abundant gem variety (Figure 8-25). Crystalline quartz, including quartz varieties of amethyst and citrine, can form in open cavities igneous and sedimentary settings, covering walls in fissures with crystal masses (Figure 8-24). Sedimentary varieties of the "quartz family" that do not display obvious crystals include chalcedony, agate, and chert.

Chalcedony is a microcrystalline form of quartz (Figure 8-27). Chalcedony is typically translucent that often displays banding or unusual botryoidal shapes. It is typically pale whitish to gray, or light blue in color.

Agate is a hard variety of chalcedony that is typically banded in appearance (Figure 8-28). The banding is due to impurities (typically iron and other metal oxides). Agate is durable and is frequently used in cabochon jewelry settings or ornamental objects. It can hold a high, durable polish.

Chert is a name for a hard sedimentary rock composed of microcrystalline quartz colored by mineral impurities (Figure 8-29). Some dull-colored varieties of chert are called "flint." Flint is chert that has a good conchoidal fracture that ancestral peoples worldwide sought after to make spear points and arrowheads. (Figure 8-30 shows flint points from southwestern Ohio). Chert forms from the precipitation of silica in lakebeds and in the deep ocean. Sources of silica include volcanic ash and siliceous skeletal remains of microscopic organisms (diatoms and radiolarians), and spicules of some siliceous sponges. Siliceous ooze dominates sediments in many places in the cold depths of open ocean basins. Exposed layered bed of oceanic siliceous deposits are called "ribbon chert" (Figure 8-31 is an example exposed on Mt. Diablo in California). Red varieties of chert are called "jasper" - a variety often used in making polished cabochons for jewelry. Chert occurs in practically any color; most color in chert are a result of traces of iron minerals (hematite is red, gray to black; limonite is yellow, orange to brown) (Figure 8-32 and 8-33). Clays, metallic oxides (copper, manganese, etc.), and organic matter add color as well. Chert is often shattered into angular fragments that can be reworked and re cemented into "chert breccia" (Figure 8-34). Note that most chert you find has been recrystallized from a softer unconsolidated silica-rich sediment or sedimentary rock, and much of what is used in jewelry making has been altered to a metamorphic variety, called metachert.
Non-clastic sediments Dissolved solids in seawaterr
Fig. 8-9. Classification of non-clastic sediments and sedimentary rocks Fig. 8-10. Dissolved matter in normal seawater; six major ions compose 99%
halite Death Valley salt pan
Fig. 8-11. Halite (rock salt) crystal masses Fig. 8-12. Salt forming in Death Valley's dry lake bed.
gypsum Selenite
Fig. 8-13. Gypsum (variety satin spar) Figure 8-14. Gypsum (variety selenite)
barite rose Fluorite crystal masses
Fig. 8-15. Barite rose crystal (Oklahoma's State Rock) Fig. 8-16. Fluorite (sometimes used in jewelry)
Calcite aragonite
Fig. 8-17. Calcite Fig. 8-18. Aragonite
shells Pearl and mother of pearl (also called nacre) are composed of aragonite
Fig. 8-19. Coral and most shells consist of calcite. Fig. 8-20. Pearl and mother-of-pearl are aragonite.
dolomite celestite
Fig. 8-21. Dolomite Fig. 8-22. Celestite
azurite rhodochrocite
Fig. 8-23. Smithsonite (greenish) and Azurite (blue) Fig. 8-24. Rhodochrocite

Quartz crystal amethyst
Fig. 8-25. Quartz crystal Fig. 8-26. Amethyst
chalcedony agate
Fig. 8-27. Chalcedony Fig. 8-28. Agate
Chert Arrowheads from southwestern Ohio
Fig. 8-29. Chert (or flint) Fig. 8-30. Flint arrowheads
Chert beds exposed on Mount Diablo, California jasper
Fig. 8-31. Ribbon chert beds. Fig. 8-32. Jasper
Chert septarian-filled veins Chert breccia
Fig. 8-33. Black chert Fig. 8-34. Chert breccia
   

Nodules, Geodes and Concretions

Many sedimentary rock formation contain "nodules." Nodules are small, round- to irregular-shaped masses of a mineral or mineral aggregate that is typically different in composition the enclosing host material. Another term for nodule is "concretion" - a rounded or oval shaped mass weathering from sedimentary rock. Examples include in pyrite nodules in shale or coal beds, chert nodules in limestone, and limestone concretions in shale. Nodules form in a variety of ways.

Nodules sometime form around decaying organic matter deposited in muddy or limey sediments. The gases released from decaying material release gases and compounds that can drive chemical reactions, causing minerals to precipitate in the microscopic pore-space in the sediments surrounding the organic matter. Cavities filled with gases or water can become sites where crystals can form. A geode is a nodule that has a hollow space inside filled with crystals (Figures 8-35 and 8-36; both geodes are from Kentucky). Common minerals inside geodes are usually varieties of quartz (clear crystal and amethyst), chalcedony, agate, calcite, dolomite, barite, fluorite, pyrite, and gypsum. Geodes can sometime have rare and unusual minerals as well. Sometimes the space can be completely filled, such as the agate nodule shown in Figure 8-37.

Septarian nodules are "limestone concretions" that have angular crack or cavities (called septa) within that are filled with crystals, typically calcite (Figure 8-38).

Nodules and concretions commonly occur in discontinuous layers or with beds of sedimentary rocks. They can have unusual shapes, such as nodules that form in sandstone (Figure 8-39). These features are secondary structures that often cut through laminations and bedding in the surrounding sediments. They also fill in voids and bubbles formed in lava flows. Some volcanic ash beds host abundant geodes.

An oolite is a sedimentary rock formed of spherical grains conposed of concentric layers, called ooids (egg stones). Once an ooid grows beyond 2 mm it is called a pisolite (Figure 8-40). Ooids and pisolites are a commonly composed of calcite or hematite, but can form from other minerals. If cyanobacteria or other microbial life is involved, they are called oncolites. Ooids, pisolites, and oncolites form as fragments of rock small shells or other organic matter roll around in agitated water, such as in a shallow lake or shallow marine embayments. Over time, they pisolites (or oncolites) grow larger and larger until they cannot move and become buried.
Kentucky geode with quartz, barite, and fluorite Geode from Kentucky
Fig. 8-35. Geode with quartz, barite, and fluorite Fig. 8-36. Geode with quartz and chalcedony
Agate-filled nodule septarian nodule
Fig. 8-37. Agate nodule Fig. 8-38. Septarian nodule
sandstone concretions Oncolites from Eocene lake deposits, Dubois, Wyoming
Fig. 8-39. Sandstone concretion Fig. 8-40. Conglomerate of Pisolites & oncolites
   

Organic gem materials and deposits

Some fossil materials make exceptional pieces for displays and adornment. They are especially coveted by people who are fascinated with Earth history. Natural history museums around the world are filled with fossil collections, and exceptional discoveries are on display, both in the paleontology collections and gemstone sections.

A fossil is a remnant or trace of an organism of a some earlier geologic age, such as a skeleton, shell, or leaf imprint, embedded and preserved in the earth's crust. Fossilization involves processes that turn plant or animal remains to stone.
A few "gems" form as organic accumulations such as when plant, shell, teeth, or bone material collects. Although fossils are generally rare, they can occur in abundance in some sedimentary environments (Figure 8-41). Gem materials from fossils include amber, jet, " nacre" (ornamental mother-of-pearl, and fossil pearls themselves). Some small fossils make attractive adornment as well.

Fossil wood is perhaps the most abundant fossil, some of which makes exceptionally colorful and durable for cutting and polishing. Small pieces are common, but whole redwood-sized trees are found in "fossil forest" beds, such as those exposed in Petrified Forest National Park in Arizona (Figure 8-42). The ancient trees (Triassic age) was buried in sediments laden with volcanic ash that supplied the silica and other minerals that replaced the wood.

In ocean sediments, the shells of ancient mollusks called ammonites are related to present day squid and octopus (Figure 8-43). They have spiral or straight conical shells composed of mother-of-pearl. Ammonites lived in the Mesozoic era and vanished along with all dinosaurs about 65 million years ago. Coral, both living and fossil, is used in jewelry as well. Other "organic gems" include, mammoth ivory, fossil invertebrates (including sand dollars, brachiopods, trilobites, etc.), bones, and shark’s teeth (Figures 8-44 to 8-46). Few of these organic materials are very durable, but they can become hardened by recrystallization and mineral replacement processes. Fossil dinosaur bone and coprolite (fossil dung) are popular gem cutting material.

Amber is fossil tree resin that oozes from trees. When buried it can harden into a transparent gem (Figure 8-47). Coal is fossil plant material. A hard variety coal called anthracite is used in the gem Jet, a black organic compound that takes a good polish (Figure 8-48). It is used in mourning jewelry (for funerals, etc).

Hunting for fossils is a popular activity for gem, rock, and mineral clubs. As urban areas are developed, construction projects expose sedimentary rocks, sometime rich in fossils. These deposits are often short lived. In addition, laws exist the make it illegal to collect certain varieties of fossils on public lands, particularly rare vertebrate materials. However, many kinds of fossils exist in great abundance and are easy to collect. Important rare and unusual fossils should be reported and donated to museum and curated educational collections.
Shell fossils preserved in sandstone Petrified wood in Petrified Forest National Park, Arizona
Fig. 8-41. Fossil shells in sandstone. Fossils can be original shell material or replaced by other minerals. Fig. 8-42. Petrified wood is abundant in Petrified Forest National Park, Arizona. Original wood is replaced with silica.
ammonite Fossil sand dollars
Fig. 8-43. Ammonite shell from a concretion Fig. 8-44. Fossil sand dollars
trilobite shark tooth from Morrocco
Fig. 8-45. Trilobite Fig. 8-46. Shark tooth
Amber Jet is anthracite coal
Fig. 8-47. Amber Fig. 8-48. Jet (coal)
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9/17/2014