Assignment 2 - Gemstone Formation & Mining Flashcards
• Understand the geological conditions necessary for gem formation. • Describe the various gem formation processes. • Define the different types of gem and mineral deposits. • Understand the political and economic factors that affect gem mining costs. • Communicate the methods used to find and evaluate gem deposits.
The surface and outermost
layer of the earth.
Crust
A layer between the earth’s
crust and its core.
Mantle
The earth’s innermost layer.
Core
General term for any
molten rock.
Magma
A section of the earth’s rigid
outer crust.
Plate
Study of the formation, structure, and movement of the plates of the earth’s crust.
Plate tectonics
Circulation in the mantle that drives the movement of the earth’s plates.
Mantle convection
A process that occurs when two of the earth’s plates collide, forcing one under the other.
Subduction
A long, widespread area in the earth’s crust where tectonic events cause major structural changes, often forming mountain ranges.
Orogenic belt
A constant formation and recycling process that creates
new rock from old.
Rock cycle
The wearing away and transport of rock materials by natural forces.
Erosion
Rock formed by the crystallization of molten material.
Igneous rock
A category of rocks that have been altered by heat and pressure.
Metamorphic rock
Rock produced from the eroded and weathered remains of existing rocks.
Sedimentary rock
Pertaining to igneous activity at the earth’s surface, where magma erupts through a volcano or fissure.
Volcanic
Crystallization of minerals from a gas.
Pneumatolysis
A “foreign crystal” that forms in unrelated rocks and is brought to the surface as a passenger
in magma.
Xenocryst
An igneous rock typically formed from cooling, once-molten granite that follows fractures in its surrounding rock.
Pegmatite
A mineral deposit that occupies an existing fissure or fracture in the rock.
Vein
Large mass of igneous rock that crystallizes underground without reaching the surface.
Intrusion
Hot, high-pressure solution that can dissolve, transport, and deposit minerals from one place to another.
Hydrothermal fluid
Changes in rock type and minerals over a wide area, caused by heat and pressure of
large-scale geological events.
Regional metamorphism
Localized changes caused by an igneous intrusion that takes place where the magma meets the surrounding rock.
Contact metamorphism
A type of metamorphism where chemical changes in the minerals and rocks result from the introduction of material from external sources, often as hydrothermal solutions.
Metasomatism
A spherical, often hollow,
mineral-lined cavity in rock.
Geode
An independent gem or mineral prospector who typically uses small-scale, non-mechanized recovery methods.
Artisanal miner
The material covering a gem deposit that must be removed
before production can begin.
Overburden
Gems found in the rocks that they formed in.
Primary deposit
Gems found away from their primary source.
Secondary deposit
A deposit where gems are eroded from the source rock but remain in place close to the
source.
Eluvial deposit
A deposit where gems are eroded from their source rock, then transported away from the
source and further concentrated.
Alluvial deposit
Workable alluvial deposit of gem minerals with economic potential.
Placer
What causes the movement of the earth’s crust?
As magma heats up deep in the mantle, it expands and rises. Eventually it cools, contracts, and falls back deep into the mantle to begin the process again. This cycle of heating and cooling, rising and falling is called
convection, and it takes place constantly within the mantle. Mantle convection creates the circulating currents that move the plates.
What geological conditions are best for gem formation?
Geological conditions during mountain building are favorable for gem formation.
What are the major rock formation processes?
- Ingneous
- Metamorphous
- Sedimentary
Which gemstones are associated with igneous rocks?
Lava erupting from fissures and volcanoes can be a source for gemstones. Moonstone can form directly in magma. Topaz and rare red beryl can crystallize in volcanic rocks from the gasses that the magma releases during the final stages of cooling.
Although gems like ruby, sapphire, peridot, and zircon are associated with volcanic rocks, scientists don’t believe they formed directly from the rock
itself. Instead, they were brought up from great depths (up to 100 km or 60 miles) as “passengers” in the surrounding magma.
What crucial role does superheated water play in gem formation?
Superheated, pressurized water can transfer elements in solution and redeposit them in veins to form gems.
Which gems are associated with metamorphic rocks?
From fabulous rubies worth fortunes to humble red garnets worth only a few cents apiece, metamorphic gems grace jewelry of every price.
The ancient mountains that run through much of eastern Africa form a huge tract of metamorphic rock that yields ruby, emerald, and alexandrite, as well as the spectacular newer gems tanzanite and tsavorite.
Which gem deposits are formed by sedimentary processes?
Opal, turquoise, and rhodochrosite are soft but beautiful gems that form close to the earth’s surface with the help of water.
What factors affect mine economics?
Mining economics are affected by demand, political climate, labor costs, environment, accessibility, type of deposit, and mining methods.
How do prospectors decide where to look for new gem sources?
An understanding of gemstone formation helps prospectors
pick the right places to look for gems.
Which gem deposits are the most economic to mine?
Secondary deposits are easier to work than primary ones and might contain greater concentrations of gems.
How does rapid vs. slow cooling affect the crystal size
In most igneous rocks, large crystals indicate slow cooling, while small crystals indicate rapid cooling.
Where do most colored stones form?
Most colored stones form in the earth’s continental crust.
Which is a key locality for hydrothermal gems?
- the Colombian emerald mines of Muzo and Chivor,
- Ouro Preto, Brazil, which has the world’s only known deposits of imperial topaz.
Which is a key locality for hydrothermal gems?
- the Colombian emerald mines of Muzo and Chivor,
- Ouro Preto, Brazil, which has the world’s only known deposits of imperial topaz.
Emeralds are rarely found in placer deposits because they’re
A.
too soft.
B.
too rare.
C.
chemically unstable.
D.
unable to withstand much abrasion.
D.
unable to withstand much abrasion.
Myanmar’s famous Mogok ruby deposits were formed by
A.
sedimentary processes.
B.
metamorphic processes.
C.
gas crystallization from molten lava.
D.
mineral-rich solutions close to the earth’s surface.
B.
metamorphic processes.
Heat and pressure transform limestone into
A. opal.
B. quartz.
C. granite.
D.marble.
D.marble.
Most of the gems in Tanzania’s Umba River Valley are found in
A. geodes.
B. placer deposits.
C. primary deposits.
D. hydrothermal gem deposits.
B. placer deposits.
Kunzite is most often found in association with
A. sapphire in lava.
B. moonstone in basalt.
C. tourmaline and beryl in pegmatite.
D. imperial topaz in hydrothermal veins.
C. tourmaline and beryl in pegmatite.
