Introduction

 The earth’s structure consists of a central core, a

mantle and a crust.
 The core has a diameter of around 7000 km, which
is nearly half the diameter of the earth.
 The core is composed of two parts, the outer
being liquid rock magma, while the solid inner
core with a diameter of 2700km, consists of heavy
rocks of iron and nickel.
The Earth  The mantle consists a semi-molten zone and has a
thickness of around 2900km. This is the important
part in the formation of gemstones.
 The thickness of the crust varies from few
kilometers to 50-60km under the sea and
mountains.
  A mineral is a substance which has been
formed within the earth by the forces of
inorganic.
 In mineralogy, there are several thousand listed
minerals, but only about 200 of these have the
Minerals and necessary qualities of beauty, rarity and
Rocks durability to make them suitable for use as gem
materials.
 Rocks are the blend of minerals. Granite, for
example, is composed of a mixture of feldspar,
quartz and mica.
Relative
Positions of rock
types in earth
crust
  The middle layer of the crust is formed by the
solidification of molten magma and these are
called as igneous rocks.
 These are sub divide into two types, volcanic
and plutonic rocks.
Igneous Rocks  The molten rock/magma erupted onto the
surface of the earth cools rapidly due to
contact with air or sea water and form volcanic/
extrusive rocks.
 When the molten rock or magma cools slowly
below the earth’s surface, plutonic/ intrusive
rocks is formed
  Result from the breaking down of earlier formed rock
masses.
 The chemical and mechanical weathering of original
rocks due to various natural processes such as rain,
wind and changing temperature cause breakdown of
rocks.
 This is subsequently transported away by moving
Sedimentary water or wind.
Rocks  The insoluble weathering products are separated into
fine and coarse particles.
 These are transported to different distances from the
source and are deposited as layers.
 Such deposits are called alluvial deposits.
 These become consolidated and cemented together to
form sedimentary rocks.
  The metamorphic rocks form when the existing
igneous and sedimentary rocks undergo
changes due to heat and pressure caused by
earth movements or when magma forces
through rocks.
Metamorphic  The rocks came into contact with magma,
Rocks change in texture and chemical compositions,
thus giving rise to recrystallization of new
minerals.
 The metamorphic rocks are usually harder than
the original rocks. (Ex: limestone → marble )
The origin of  The origin of a gem is its place of genesis in the Earth’s
crust or mantle. Except for organic substances such as
gems amber and jet
 Sedimentary rocks can’t contain primary gem
materials.
 However, if the pre-existing weathered rocks contained
heavier minerals , these may have been washed out
and swept away to form secondary or alluvial deposits.
 These gemstones often end up as water-worn pebbles,
and can be seen in the gem gravels of Brazil, Myanmar
(Burma) and Sri Lanka
  Most of important gem minerals, such as feldspar and
quartz, tourmaline, beryl, topaz and zircon, originated
in intrusive or plutonic igneous rocks
 Because slower rate of cooling in the middle and lower
parts of the crust made it possible for quite large
The origin of crystals to form from the original molten residues.
gems (Cts)  Many of the intrusive gem–bearing rocks formed as
coarse-grained granites called pegmatites.
 Geodes also originated in igneous rocks in which quartz
and other minerals have been precipitated as crystals
in almost spherical cavities.
  The chemical reactions generated in metamorphic
rocks when molten magma was forced into cooler
rocks created the gemstone varieties of emerald,
The origin of alexandrite, ruby and sapphire.
gems (Cts)  Other gem minerals were formed as a result of the
large-scale shearing and crushing of rocks. Examples of
these are garnet, andalusite, serpentine, nephrite and
jadeite.
 Diamonds differ from the rest of the gem minerals in
that they were formed somewhere in the region
between the lower part of the Earth’s crust and the
beginning of the mantle.
 Diamonds crystallized at least 120 km below the
Earth’s surface from carbon (in the form of carbon
dioxide or methane) at very high temperatures and
pressures
  Most often gem bearing gravel found as secondary
Secondary alluvial deposits.
Deposits  There can discovered in the area of flood plains or
along the courses of ancient dried-up river beds.
 Sometimes the gem-bearing gravels are found nearer
the surface in the beds of existing rivers. Where there is
a bend in a river
 water flow on the inside of the bend will slow and at
this point denser minerals such as gem materials and
precious metals will sink and form a placer deposit.
 If the river meanders, forming an ‘ox-bow’, a channel
may be cut from bank to bank to divert the river.
Dense minerals are
deposited where river
water slows around the
inside of a bend
Recovering gem
gravels from an ‘ox-
bow’ river meander
The major gem  The locality of a gem deposit is the country or area in
localities which it is found.
 Sri Lanka is host to a wide range of gem varieties
including ruby, sapphire, the more common varieties of
garnet, chrysoberyl (alexandrite), quartz, moonstone,
spinel, topaz, zircon, tourmaline, andalusite and
sinhalite.
 The Malagasy Republic has aquamarine and the pink
variety of beryl (but not emerald), many of the varieties
of quartz and garnet, plus topaz, tourmaline,
orthoclase feldspar and the kunzite variety of
spodumene.
 Myanmar(Burma) and Thailand (Siam)
famous for sapphire and ruby deposits.
The major
gem localities In South America there are the famous
emerald deposits of Columbia and Brazil,
the latter also being a prolific source of
amethyst, aquamarine and topaz
  The recovery of gems from the Earth’s crust varies from
Gem Mining simple manually-intensive methods to highly
Techniques mechanized operations.
 The mining of the alluvial gem deposits in Myanmar, Sri
Lanka, Thailand and Brazil consists of sinking round or
square section pits of up to 15 meters deep until a layer
of gem gravel is reached.
 The gravel is dug out and hauled or winched to the
surface where the gem content, which is more dense
than the sand and silt, is separated out by a rotary-
motion hand-washing operation using a shallow woven
basket
  Colour
 Colourless – Diamond
 Maximum Colour – Aquamarine
 Optimum Colour – Sapphire , Ruby
 Colour Change – Alexandrite

Essential  Clarity
 Transparent/Clean – Faceted
Qualities of a  Translucent/ Opaque -- Cabochon
 Special Optical Effects
Gem Minerals  Chatoyancy – Cat’s Eye
1. Beauty 

Asterism – Star Sapphire
Play of Colour – Opal
 Adularacence (milky, bluish luster or glow originating from below the
surface of the gemstone) -- Moonstone
 Irridescence – Fire opal
 Labradorescence (Effect that causes a mineral to display dark, metallic-
like color shimmers, usually blue and green) – Labradorite
Chatoyancy
Asterism
Play of Colour
Play of Colour
Adularescence
Irridescence
Labradorescen
ce
Essential  Depend on
 Supply and Demand
Qualities of a  Fashion – Amber and Pearl
 Natural vs Artificial
Gem Minerals
2. Rarity
  Durability or Hardness is resistance to day today wear, chemical
attack, sweat etc.
Essential  Old classification (depend on Moh’s Scale)
 Precious Stones ( 8 – 10)
Qualities of a  Semi Precious Stones (1 – 8)
Gem Minerals  But Emerald (7.5), Opal (5.5) are not semi precious.
3. Durability  Topaz (8) is not precious.
 Therefore terms precious and semi precious are not used in
gemology anymore.
 Moh’s Hardness Chemical Composition Absolute Hardness
1 Talc Mg3Si4O10(OH)2 1
2 Gypsum CaSO4·2H2O 3
3 Calcite CaCO3 9
4 Fluorite CaF2 21
Moh’s 5 Apatite Ca5(PO4)3(OH−,Cl−,F−) 48
Hardness 6 Orthoclase feldspar KAlSi3O8 72
Scale 7 Quartz SiO2 100
8 Topaz Al2SiO4(OH−,F−)2 200
9 Corundum Al2O3 400
 10 Diamond C 1600