Jewel__A_Celebration_of_Earth_s_Treasures

TREASURES OF THE EARTH 013

Created through organic processes, organic

gems are not commonly crystalline. Jet is

a form of coal, derived from plant matter;

coral and shell are the secreted skeletons

of marine animals, while pearls are formed

by shelled molluscs; and amber and copal

are tree resins, fossilized and semi-fossilized.

Organic gems are generally softer than

minerals and easier to work.

A crystal is a solid, the component atoms of

which are arranged in a particular, repeating,

three-dimensional pattern. When these

internal patterns produce a series of external,

flat faces arranged geometrically, a crystal,

such as the rhodochrosite above, is created.

Polished copper and

silver in a rock matrix

Stalactitic amber

Gem deposits Gems such as rubies (red

corundum) and sapphires (corundum in other

colours) can be recovered from the rocks in

which they form, or from placer deposits.

Gems in the Earth

The origins of rocks, minerals, and gemstones

Rocks and minerals are created in the rock cycle. All rocks begin as igneous,

but over time they are altered by re-melting, erosion, or metamorphosis –

weathering and erosion lead to the formation of sedimentary rock, which can

turn into metamorphic rock through temperature or pressure conditions.

GEMS IN NATURE

Mineral gemstones are directly mined from the rocks in which

they were originally formed (see p.25): examples of these

include diamond, tanzanite, ruby, kyanite, celestine, emerald,

tourmaline, and aquamarine. Other types of gem, released

from their original rock by weathering, can be mined from

placer deposits found in stream gravels. Examples of these

varieties include topaz, sapphire, chrysoberyl, garnet,

zircon, and spinel.

Magma

Igneous

rock

Pressure

and heat

Pressure

and heat

Metamorphic

rock

Sedimentary

rock

The rock cycle This

diagram shows how different

types of rock are formed.

Crystals

Organic gems

These are chemical elements that occur in

nature uncombined with other elements (see

also p.14), including gold, silver, and diamond

(carbon). With the exception of platinum

and gold, most metals are extracted from

minerals that contain them.

Native elements

Transportation

Weathering

Uplift to

surface

Heat and

increased

pressure

Burial,

sedimentation Transportation

Weathering

Rock veins

containing corundum

(sapphire, ruby)

Rock veins

containing

corundum

(sapphire, ruby)

Stream

Placer deposits with

concentration of heavy

sapphire and ruby

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