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Chapter 6 - Gems From Igneous Rocks

This chapter examine selected common gemstones derived from rocks of igneous origin. Some gems, like diamonds and peridote, melt at very high temperatures and can be transported by flowing magma (molten rock and gases) from sources deep in the earth, including the lower crust and mantle. Some varieties of gems (including tourmalines, garnets, and beryls) form when pockets of water-rich magma cools and forms crystal masses called "pegmatites." Still others, particularly varieties of quartz gemstones, form during or after magma or lava cools and fractures in the cooling rock become locations where hot fluids (water and gases) release dissolved minerals to form crystal masses lining cavities in the host igneous rock.

This chapter examines the character of igneous rocks, including varieties of intrusive or plutonic rocks (where magma crystallizes below the surface) and extrusive or volcanic rocks (where magma reaches the surface and erupts), how and where they occur, and how to recognize landforms and rock types associated with volcanic areas where gemstones may occur.
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The Rock Cycle
Fig. 6-1. Igneous rocks include intrusive (plutonic) and extrusive (volcanic) varieties.

The Igneous Origin of Diamonds

Diamonds are a rare occurrence on the surface of the planet because it takes extremely hot and high pressure conditions to create them. Physical and chemical conditions where diamonds form only exist in the mantle, nearly 70 miles down or more. In that environment in the upper mantle, diamonds may be a common mineral! It takes incredible events, nothing that has ever been witnessed in historic times, to bring diamonds to the surface. Diamond deposits around the world (that have any economic significance) are associated with volcanic features called diatremes (Figure 6-2). A diatreme is a long, vertical pipe formed when gas-filled magma forces its way through the crust to explosively erupt at the surface. Kimberlite a special kind of intrusive igneous rock associated with some diatremes that sometimes contain diamonds, typical coarse grained an bluish in color. Diamonds are xenoliths carried up from deep sources in the mantle, and often occur in association with other gem minerals including garnet, spinel and diopside. Most "economically significant" diamond deposits occur in ancient rocks (Precambrian age), but have been discovered on all continents. Because diamonds are so hard, they survive torturously-long histories, recycled through sedimentary and metamorphic environments without being destroyed. As a result they have been found almost everywhere as very rare, isolated discoveries. Diamonds of microscopic size have been discovered in meteorites and asteroid impact sites, and some metamorphic rocks. They are most extensively mined from kimberlite pipes or from alluvial gravels derived downstream from diamond source areas. It should be noted that most diamonds are not of gem quality, but those that are not are used for industrial purposes.
kimberlite
Fig. 6-2. Diamond-bearing kimberlite pipes are diatremes that originate in the mantle.

Pegmatites

Pegmatite is a coarsely-grained crystalline igneous rocks with interlocking crystals typically several centimeters in length (or larger, including the world’s largest crystals, some larger than 10 meters in length). many exotic and important minerals, including many gemstones, are found in them (Figures 6-3 to 6-5).

Mineralogy: Most pegmatites are granitic in composition, having granite’s constituent minerals quartz, feldspar (Na-plagioclase and orthoclase) and mica, commonly muscovite. Less common are gabbroic pegmatites—a gabbro with very large crystal grains of amphiboles, biotite, and some pyroxenes.

Occurrence: Pegmatite typical form as masses in igneous dikes and veins. They are most common along the margins of large igneous intrusions, especially in regions associated with batholiths. Some pegmatites apparently form from very fluid remnants of cooling magma with incompatible elements that are driven off of the main cooling plutonic rock body. An aureole zone is where host rocks show the effects of being nearby cooling large intrusions. Fracture formed in wall rocks surrounding a nearby intrusion can be flooded with late-stage magma and high-pressure fluids derived from magma that are rich in volatile components including water, CO2, and other gases. The volatile components both lower the temperature that mineral can form, but also enhance diffusivity (the ability for elements to move around within a fluid to interact with crystal nucleation sites). This diffusivity allows large and rapid crystal growth—allowing minerals to separate into pockets, some with rare and unusual compositions. These late stage fluids can be enriched in the elements necessary for important gems. Not all pegmatites form the same way.

Pegmatites frequently occur in association with aplite dikes and veins (Figure 6-6 to 6-7). Aplite is a light-colored granitic rock composed of quartz and feldspar with sugary texture. Pegmatites occur in pod-shaped mass, usually as small pods to long linear zones, in some rare cases hundreds of feet in size and thickness. Some pegmatites may contain open pockets (called vugs) that are typically surrounded by well-formed crystal masses radiating into the open voids.

Some pegmatites are best described as metamorphic in origin, formed as rocks rich in fluids only begin to melt and separate from host rocks into isolated pockets and veins (discussed with metamorphic rocks).

Economic Value: Gem minerals found in pegmatites include: apatite, beryls (including emerald), cassiterite, corundum (sapphires), feldspars (including aquamarine and perthite), fluorite, garnet, lepidolite quartz varieties (crystal, rose, and smoky), spodumene, topaz, and tourmaline. Pegmatites sometimes containing rare minerals rich in uncommon elements, including important rare-earth elements. Pegmatite ore around the world have been mined for such economically strategic elements, primarily for beryllium and lithium, but also aluminum, bismuth, boron, cesium, molybdenum, niobium, potassium, tantalum, thorium, tin, tungsten, and uranium.

pegmatite schorl
Fig. 6-3. Pegmatite dike in granite displaying large crystal of amphibole and feldspars (orthoclase and plagioclase). Fig. 6-4. Pegmatite with schorl (black tourmaline) in quartz and orthoclase feldspar from Black Hills, South Dakota.
Tourmaline-bearing pegmatite from San Diego County, California Pegmatite dikes at Kehoe Beach, Point Reyes National Seashore, California
Fig. 6-5. Pegmatite with watermelon tourmaline. minerals: pink (elbaite) and green (liddicoatite) from from San Diego County, California Fig. 6-6. Pegmatite and aplite dikes and veins in granitic rocks on Kehoe Beach, Point Reyes National Seashore, California
Pegmatite vein exposede in a boulder in Death Valley, California Gabbro pegmatite from Aromas Quarry, California
Fig. 6-7. A boulder of with aplite and pegmatite veins in Death Valley National Park, California. Fig. 6-8. Gabbro pegmatite (crystals of amphibole and plagioclase) from Aromas Quarry, Monterey County, CA
   

Many Gems of Igneous Origin Are Silicates

A silicate is a mineral that contains silica (SiO2) within its crystal structure. Silica content refers to the total content of (SiO2) in a rock or mineral. For instance, a rock composed entirely of albite (a feldspar with the chemical formula NaAlSi3O8 has a silica content of 77%. Pure quartz (mineral) has 100% silica (Figure 6-9). Although silicate minerals are abundant and important not all that many gems are silicates. Gem minerals that are silicates include varieties of beryls (including emerald), feldspars, olivine (including peridote), quartz, garnets, topaz, tourmalines, and others. A lot of gems, even in of silicate family, have exotic elements. These exotic elements, such as beryllium, boron, lithium, chromium, vanadium, and zirconium need to be concentrated by changing physical and chemical conditions associated with the molten material and fluids that form them. So gem formation often requires unusual geologic circumstances. Sometimes water-rich hot fluids generated from the cooling molten material and rock separate from the main magma and bring with them the exotic elements that do not easily fit into the common silicate mineral crystal structures (that includes ferromagnesian silicates, quartz, feldspar, mica, etc.).

Quartz is one of the most abundant minerals in the crust. It is the most abundant in "gem" so all varieties are considered "semiprecious."

Quartz has a variety of forms that are used a gems or precious stones. Pure, clear quartz is called "rock crystal" (Figure 6-9). The combination of trace impurities and exposure to radiation give other varieties of quartz their color. Smoky quartz gets its dark color from silicon atoms set free in the crystalline structure of quartz (Figure 6-10). Amethyst, citrine, and rose quartz have traces of iron and other metals in the crustal structure and are also discolored from natural radiation exposure in the environments where they form (Figures 6-11 and 6-12). Heating colored varieties of quartz can undue the effects of radiation exposure and alter the color of the mineral. Most commercially sold citrine gems are actual heat-treated amethyst or smoky quartz.

Quartz has different crystal structures. The most common, called α-quartz, is stable in the pressures and temperatures of the surface environment. At at 573 °Celsius it undergoes a reversible change in crystal structure to form β-quartz. Over time, if conditions are right, β-quartz converts to α-quartz. Silica (Si02) has a polymorph (an alternative crystal structure) called moganite. Quartz and moganite combine in microcrystalline size to form the mineral chalcedony (Figure 6-13). Different varieties of quartz combine to form different kinds of stones that show contrasting bands or patches of color. These include agate, sard, onyx, carnelian, heliotrope, and jasper. However, most of these forms also occur in sedimentary or metamorphic settings, not entirely igneous. For instance, fossil wood is frequently preserved as brightly colored varieties of quartz and is abundant in deposits rich in volcanic ash (Figure 6-14). Chemical interactions between the volcanic ash, groundwater, and wood buried in the volcanic ash replace the organic tissue of the wood with silica (in various forms of silica and other minerals).
Quartz varieties with different trace impurities
quartz crystal smoky quartz
Fig. 6-9. Rock crystal is pure quartz. Quartz is one form of silica (SiO2) Fig. 6-10. Smoky quartz is dark from radiation exposure
Amethyst rose quartz
Fig. 6-11. Amethyst gets it color from both traces of iron and radiation exposure over time. Fig. 6-12. Rose quartz gets its color from traces of titanium, iron, or manganese.
chalcedony Fossil wood as onyx
Fig. 6-13. Chalcedony is a variety of microcrystalline quartz with a botryoidal appearance. Fig. 6-14. Fossil wood can be wood replaced by silica and other minerals derived from volcanic ash.

Volcanoes and Volcanic Features

A volcanic cone is a hill or mountain formed by the accumulation of volcanic material around a vent where magma reaches the surface (Figure 6-15). The size and shape of a volcanic cone depends on the volume of material ejected over time, the composition and temperature of the material (lava, rock and gases) vented from the volcano, and the nature of the eruption (explosive or otherwise). There are a variety of different kinds of volcanoes.

As discussed above, different plate-tectonic settings produce different kinds of volcanoes (Figure 6-16). Most volcanoes of the world are associated with plate boundaries, either divergent boundaries (spreading centers) or convergent boundaries (volcanic arcs associated with subduction zones) (see Figure 5-25 in Chapter 5).

Volcanic eruptions release a variety of material including lava, tephra (ash, cinders and rock fragments), gases, and water. Volcanic eruptions can be gentle venting of fluid lava to catastrophic explosions that blow volcanoes apart and scatter ash and material over large areas (Figures 6-17 and 6-18).

In general, very hot magmas produce more gentle eruptions that vent gases continuously where they erupt and pour lava on the surface that can flow under the influence of gravity over long distances. Hot magmas occur in association with spreading centers and hotspots. These hot magmas are mafic to ultramafic in composition.

Cooler magmas associated with subduction zones tend to be rich in dissolved water and gases. As this magma approaches the surface, the water and gas trapped in the cooling magma escapes producing great pressure that expands explosively causing tremendous eruptions of clouds of steam and ash (Figure 6-19). Most of the material falls to the earth in the vicinity of the eruption piling up to build up tall volcanic cones. These eruptions can occur suddenly and be tremendous, blowing entire mountain-size volcanoes apart, only to be rebuilt by later eruptions. Explosive volcanoes tend to be felsic to intermediate in composition.

Features Associated with Volcanoes and Volcanism

Lava flows are lava flowing on the surface under the influence of gravity. The term lava flow also applies to a deposit of volcanic rock formed from lava flowing and cooling on the land's surface (Figure 6-20).

A cinder cone is a cone-shaped hill formed around a volcanic vent by fragments of lava (blocks, cinder, ash) blown out during eruptions (Figure 6-21). Cinder cones and lava flows can for from a vent or series of vents associated with an eruption (Figure 6-22).

A shield volcano is broad, domed volcano with gently sloping sides, characteristic of the eruption of fluid, basaltic lava. The large volcanoes on Hawaii are shield cones (Figure 6-23). Shield cones are formed from very hot lavas.

A composite cone (also called stratovolcano) is a typically tall and large, steep volcanic cone built up of many layers of both lava and pyroclastics (tephra, pumice, and volcanic ash), often created by a series of cyclic eruptions in which pyroclastics are created by explosive eruptions until the vent is open, then lava flows occur (Figure 6-24). Most large continental volcanic cones are this type.

A dome volcano is a volcano composed of lava domes; a lava dome is a roughly circular mound-shaped protrusion resulting from the slow extrusion of viscous lava from a volcano. Lava domes can vary from basalt to rhyolite in composition although most preserved domes tend to have high silica content (Figure 6-25).

A fissure eruption is a volcanic eruptions along rift fault zones that can flood large area with basalt flows (Figure 6-26). In prehistoric times, great fissure eruptions have occurred. Flood basalts are the result of a giant volcanic eruption or series of eruptions that coats large stretches of land or the ocean floor with basalt lava. Another older name is trap basalt.

A crater is a large bowl-shaped vent or collapsed top of a volcano created by explosive eruptions (Figure 6-27). A crater may also be a large bowl-shaped hole created by an meteor or asteroid impact and explosion.

A caldera is a very large volcanic crater, typically one formed by a major eruption (explosion) or the inward collapse of a volcanic cone following an eruption (Figures 6-28 and 6-29).

Large scale plutonic features:
these features are typically extensive, measurable or mapable in the range of tens to hundreds of miles in size.

A batholith is a great mass of igneous rock, extending to great depths, formed from extensive magmatic intrusions (plutons) over a long period of time and throughout a region, typically associated with volcanic arcs.
For example, the core of the Sierra Nevada Range in California is the exposed remnant of a great batholith (Figure 6-30). It is nearly 400 miles long and up to 70 miles wide.

A volcanic field is an area of the Earth's crust that is prone to localized volcanic activity. They usually contain tens to hundreds of volcanoes, vents and lava flows. Volcanic fields may have hundreds of vents forming volcanoes and lava flows intermittently over long periods of time (millions of years). An example is the San Francisco Volcanic Field in northern Arizona (Figure 6-31). Volcanic fields may be associated with plate boundary regions or hotspots in the mantle being overridden by continental crust (as in Yellowstone, Figure 6-32) or oceanic crust (as in Hawaii, Figure 6-33). A hotspot is a place in the upper mantle of the Earth at which extremely hot magma from the lower mantle upwells to melt through the crust (oceanic or continental) usually in the interior of a tectonic plate to form volcanic features on the surface. Examples include the Hawaii or Yellowstone hotspots. (Figure 6-32). A volcanic arc is a generally curved or linear belt of volcanoes above a subduction zone, including the volcanic and plutonic rocks formed there.
Types of volcanoes
Fig. 6-15. When magma reaches the surface it becomes lava. Fig. 6-16. Types of volcanoes are based on shape of their cones.
Pu'u'o'o Volcano erupting on Hawaii volcanic
Fig. 6-17. Gas and lava venting at Pu'u'o'o Volcano on Hawaii. Hawaii volcanoes produce some of the hottest lava on Earth. Fig. 6-18. Explosive volcanic eruptions like this one in the Aleutian volcanic chain are associated with relatively "cool" and "wet" magmas.
Lava flowing in Hawaii Volcanoes National Park
Fig. 6-19. When magma containing dissolved water and gases is released in a volcanic eruption it expands hundreds of times in volume creating ash-filled clouds. Fig. 6-20. Hot flowing lava on Hawaii. The lava behaves predictably enough for visitors to approach close enough to view basalt rocks and landforms forming.
Cinder Cone in the Mojave National Preserve, California cone and lava flow of SP Crater, Arizona
Fig. 6-21. Cinder Cone
Mojave National Reserve, CA
Formed from gaseous lava eruptions producing a cone composed of ash and cinders (tephra).
Fig. 6-22. Airliner view of SP Crater, a cinder cone and lava flow in NW Arizona. The cinder cone formed around a vent, lava flowed from the base of the cone.
Halualai volcano is a shield cone on Hawaii Mt. Shasta
Fig. 6-23. Shield Cone
Halualai Volcano, Hawaii
Gentle slope formed from numerous hot fluid basaltic lava flows over time.
Fig. 6-24. Mt. Shasta is a large, complex stratovolcano located at the southern end of the Cascade range in northern California.
Lava dome in Mount St. Helens crater Fissure eruption
Figure 6-25. Lava dome forming in the crater of Mount St. Helens in Washington.
Figure 6-26. Fissure eruption along a rift zone on Mauna Loa on Hawaii.
Halemaumau Crater on top of Kilauea Volcano on Hawaii Crater Lake in Oregon is a small caldera relative to the massive scale of the one in Yellowstone.
Fig. 6-27. Halemaumau Crater on top of Kilauea Volcano on Hawaii. The crater is one of the vents on the volcano, sometime filling with a lava lake, rising and sinking with different phases of the erupt. Fig. 6-28. Crater Lake in Oregon is a small caldera about 2 miles in diameter. It formed from the eruption of Mt. Mazama about 9000 years ago. It is now hosts the deepest lake in North America.
Yellowstone caldera Sierra Nevada Batholith San Francisco volcanic field Yellowstone Hotspot migration over the past 18 million years Hawaii's hotspot
Fig. 6-29. Map of showing the extent of the caldera in Yellowstone National Park, Wyoming. A massive eruption about 2 million years ago ejected rhyolitic material followed by basalt flows filling in crater. Fig. 6-30. The Sierra Nevada Range in California is a great batholith exposed by erosion. In Cordilleran volcanic arc formed during the Mesozoic Era (~200 to 80 million years ago) Fig. 6-31. San Francisco Volcanic Field covers 1,800 square miles in northern Arizona. Over the last 7 million years eruptions from about 600 vents formed hundreds of cones and lava flows.
Fig. 6-32. Map show the migration of the Yellowstone Hotspot across the western United States over the past 16 million years. Numerous large calderas and volcanic fields formed as the hotspot slowly migrated eastward. Fig. 6-33. The Hawaiian Hotspot is a deep-seated igneous plume rising beneath the overriding Pacific Plate. The hotspot has been active of 80 million years, forming the Emperor Seamount Chain.

Landforms Associated With Ancient Igneous Rocks

Although active volcanoes may have rich mineral deposits associated with them, they are not places that any safety-conscious person might want to dig a mine! On the other hand, ancient igneous rocks (plutonic and volcanic) have recognizable landforms that can be sites for gem exploration (Figure 6-34). Over time, erosion strips away materials on the surface, striping away exposing volcanic and plutonic features that were once deeply buried.

Intermediate- to small-size igneous features: these are features associated with ancient plutonic activity (and now exposed by erosion of the landscape). Individual plutons commonly have a dome-like shape when exposed by erosion (Figure 6-35). A stock is an igneous intrusion having a surface exposure of less than 40 square miles, differing from batholiths only in being smaller (Examples: Figures 6-36 and 6-37). Circular or elliptical stocks may have been vents feeding former volcanoes.

A laccolith is a lens-shaped mass of igneous rock that has been intruded between rock layers creating a dome-shape chamber filled with igneous rock (Figure 6-38).

A dike in a vertical or near vertical wall of igneous rock formed where magma squeezed into a fault zone before crystallizing. Dike form in volcanic regions, and often appear as dark castle wall-like features on landscapes where the host rock surrounding the intrusion have eroded away. A sill is a tabular, typically more horizontal than vertical, sheet of intrusive igneous rock that has intruded between layers of older rocks (Figure 6-39). Dikes and sills can form simultaneously of at different stages in igneous activity in an area. Dikes and sills can be small to massive is in size (Figures 6-40 and 6-41).

Columnar jointing in Devils Postpile National Monument, California Columnar Jointing forms where magma or lava pooled at or near the surface shrinks as it cools. This shrinkage causes a great amount of stress within the cooling body of material, causing it to fracture in polygonal shaped columns perpendicular to the stress.
Fig. 6-42. Columnar joints exposed in an old lava flow in Devils Postpile National Monument, California
Volcanic ash beds in John Day Fossil Beds National Monument Ash Beds are layered deposits of materials ejected from volcanic eruptions. Successive volcanic eruptions in a region can accumulate in basins. Individual beds can sometimes be traced over long distances across a region and can range in thickness from very thin to massive, hundred of feet thick.
Fig. 6-43. Massive ash beds exposed in John Day Fossil Beds National Monument, Oregon.
features associated with volcanism Plutons of Ryans Peak in Joshua Tree National Park, California
Fig. 6-34. Modern and ancient landscape and geologic features associated with volcanism.
Fig. 6-35. Dome-like tops of granitic plutons in Joshua Tree National Park, California
Devils tower in Wyoming is a stock Shiprock, a stock with radiating dikes
Fig. 6-37. Shiprock in northwest New Mexico is a stock with radiating dikes.
Fig. 6-36. Devils Tower in Wyoming is an eroded remnant of a stock
Bear Butte, South Dakota dikes and sills
Fig. 6-38. Bear Butte in South Dakota is a classic example of a laccolith. Fig. 6-39. dikes (vertical) and sills (horizontal) in Black Canyon, Arizona
Palisades Sill along the Hudson River, New Jersey Palisades sill in the Grand Canyon
Fig. 6-40. The Palisades along the Hudson River in New Jersey (across the river from Manhattan) is a massive sill nearly 50 miles long. Fig. 6-41. Another great sill (also called the Palisades Sill) is exposed along the Colorado River in the Grand Canyon.

Volcanic rocks with unusual textures

Materials ejected from volcanic eruptions have some unique characteristics. Some are of interest to the gem community, not that they are gems or considered precious, but they can be cut or shaped into interesting variety of uses for jewelry and art. Visitors to Hawaii are introduced to the terms pahoehoe (pronounced "pā-hoi'hoi') and aa (ä’ä) (Figures 6-44 and 6-45). Pahoehoe has a ropey fluid texture formed when hot basaltic lava cools quickly. Aa is lava rock with a rough, blocky surface when a lava flow continues to move slowly as it cools, and congealed rock breaks into rough pieces. People who walk on it barefoot frequently yell "Ah! Ah!" (Old Hawaiian joke).

Vesicular lava rock is any igneous rock that has gas bubbles trapped in a fine-grained volcanic rock. Scoria is volcanic rock with a light, frothy consistency due to the high volume of gas bubbles trapped in the rock as it cools as lava is ejected from a volcano (Figure 6-46). If the rock is so frothy from trapped gas inside that it will float it s called pumice. Huge mats of pumice have been observed floating on the ocean after massive volcanic eruptions.

Tuff is a volcanic rock that contains an abundance of visible fragments of volcanic rock that have been crushed or welded together by the heat released during an explosive volcanic eruption (Figure 6-47).

Flow banded lava rock is a volcanic rock that has a layered appearance due to flowing or stretching (like taffy candy) that formed as the lava was still flowing as it cooled (Figure 6-48).

Obsidian is a dark, glasslike volcanic rock formed by the rapid solidification of lava without crystallization (natural glass)(Figure 6-49). Obsidian breaks with a conchoidal fracture like glass. Bubbles in volcanic rocks can fill with minerals, including gem minerals. Snowflake obsidian is very attractive when tumbled or polished. The snowflakes in the obsidian are crystals (phenocrysts) of feldspar.
Pahoehoe lava Aa lava, Hawaii
Fig. 6-44. Pahoehoe lava has a ropy texture (Hawaii). Figure 6-45. A'a lava has a rough, blocky texture (Hawaii)
scoria volcanic tuff
Fig. 6-46. Scoria Fig. 6-47. Volcanic tuff
Andesite Obsidian
Fig. 6-48. Flow-banded lava rock display lines where the partly molten lava stretched like pulling taffy. Fig. 6-49. Obsidian (natural glass) is usually black but can occur in a variety of colors (from Glass Mountain, CA)

Hydrothermal Features and Deposits

Igneous activity releases fluids (water and gases). Heat from hot material also heats the groundwater. As fluids move, they can dissolve and precipitate minerals as the physical and chemical conditions change. Where water (steam) and gases vent at the surface they form fumaroles, hotsprings and geysers.

A fumerole is an opening or vent in or near a volcano, through which hot sulfurous gases, steam, and other gases emerge (Figure 6-50).

A hot spring is a spring that is produced by the emergence of geothermally heated groundwater from the Earth's crust, and usually defined as spring water warmer than the human body (Figure 6-51). If they are cooler than body temperature but warmer than average air temperature it is called a warm spring.

A geyser is a hot spring in which water intermittently boils and erupts, sending a tall column of water and steam into the air (Figure 6-52).

Hydrothermal vents occur on the seafloor and are best known from locations on or near mid-ocean ridges. Under the great pressures at the bottom of the ocean water will not turn to steam. Fluids venting from the seafloor have been measured at temperatures hot enough to melt glass! These fluids are rich in a variety of metals and rare elements that precipitate around the vents called "black smokers." (Figure 6-53). Ancient black smokers and related deposits are associated with many ore deposit now found on continents.

Hydrothermal veins are fractures in rock that have been filled with minerals (most commonly quartz and/or calcite) precipitated from groundwater or hot fluids of magmatic origin (Figure 6-54). Hydrothermal veins are transitional to cooler setting that those associated with pegmatite. Open pocket can also be filled with gem minerals and metals ores including gold, silver, and copper. Most of the world's copper come from large copper porphyry deposits (Figure 6-55). These copper-rich deposits contain precious stone and minerals including azurite, chrysocolla, malachite, turquoise, and others. Cinnabar (mercury ore) also forms in hydrothermal deposits. Hydrothermal deposits are also associated with sedimentary and metamorphic environments.

Some gems form after beds of ash and lava settle because the ash is soluble in the hot fluids that may rise from hydrothermal (hot water) vents emanating from below. Hot gases also rise and can carry mineral forming elements. Because the hot water is filled with dissolved minerals, on cooling a precipitation of crystals into spaces in the rock may occur as the hot gases and water rise and cool. Some opals and nodules such as thunder eggs form in this way (Figures 6-56 and 6-57). Geodes are small crystal-filled cavities, usually with hard, typically with spherical or ovoid outer surfaces. Although geodes occur in igneous deposit, most are of sedimentary origin. Any open gas pocket can become a void filled with crystals if conditions are right.
Fumerole, Hawaii Hotspring, Yellowstone
Fig. 6-50. Fumerole with sulfur deposits, Hawaii Fig. 6-51. Boiling hot spring, Yellowstone, Wyoming
Geyser, Yellowstone black smoker deposits on the seafloor
Fig. 6-52. Grand Geyser erupting in Yellowstone National Park, Wyoming Fig. 6-53. Hydrothermal veins in North Cascades National Park, Washington
Quartz veins Copper porphyry
Fig. 6-54. Black smokers on the seafloor deposit minerals Fig. 6-55. Copper porphyry from Arizona
Opal thunder egg
Fig. 6-56. Precious opal is one variety of opal. Fig. 6-57. Thunder egg (with banded agate).

Gems of Igneous Origin

Gems are minerals that can form under limited physical and chemical environments. In some cases, these may be only igneous settings. However, some minerals, quartz as an example, can form in a variety of settings, igneous, sedimentary, and metamorphic. Below is a chart showing selected gems formed in igneous settings.

Selected Gem Minerals From Igneous Environments

Xenoliths (X)
Phenocrysts (F)
Pegmatites (P)
Crystalline Plutonic (C)
volcanic (V)
hydrothermal (H)
other geologic settings (O)
apatite P O   corundum X O     peridote X       spinel X P C  
augite X F C - ruby X O     prehnite V O     topaz P O    
beryls P O   - sapphire X O     quartz varieties P V H O tourmalines P O    
- aquamarine P O   diamond X       - agate V H O   - achroite P O    
- emerald P O   feldspars F P C O - amethyst P V H O - elbaite P O    
- morganite P O   - amazonite F P C O - ametrine P V H O - liddicoatite P O    
chrysoberyl P     - labradorite F P C O - bloodstone V O     - rubellite P O    
cinnabar H     - moonstone X P O   - chalcedony V O     - schorl P O    
copper minerals H O   fluorite P H O   - citrine P V H O zircon

X C O  
*azurite H O   garnets X P O   - crystal P V H O          
*chrysocolla H O   spodumene
P       - rose P                
*malachite H O   - kunzite P       - smoky quartz P O              
*turquoise H     opals H O     snowflake obsidian V                
Links go to Smithsonian Institution's "Gem Gallery" website
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