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Introduction to Gemology
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LECTURE FOR UNDERGRADUATE
STUDENTS
INTRODUCTION TO GEMOLOGY
Dr. Osama Shaltami
Department of Earth Sciences
Faculty of Science, Benghazi University, Libya
Introduction
 Nearly 4000+ varieties of minerals are identified in
nature; only a few are referred as gem minerals
due to their exceptional qualities and properties.
 Gems have been a part of human history for more
than 5000 years. Throughout the ages gemstones
have been seen as representations of wealth, power
and symbols of supremacy.
 Gem minerals are usually hard and relatively free
from cleavage and occur as transparent crystals, with
few exception such as opal, fluorite etc.,
 Thus gemstones are artificially polished fragments
(faceted or with smooth curved finish).
 The study of gemstones is an accepted part of
Mineralogy .
 A gem is a naturally occurring material
desirable for its beauty, valuable in its
rarity, and sufficiently durable to give
lasting pleasure.
 Gemology- is the subject matter the
scientific study of gemstones (from its
genesis to valuation).
 Lapidary – is the science of cutting and
polishing a gem mineral.
 Over the centuries numerous magical and
medicinal properties were attributed to precious
stones.




In Ayurveda (Indian System of Medicine)
Audbhida-Extract form Plants,
Jangama-Extract from Animals
Parthiva-Extract from Metals and Minerals
Different types of BHASMA such as Hirka Bhasma
(Diamond), Manikya Bhasma (Ruby), Swarna
Bhasma (Gold), Abhraka Bhasma (Mica) etc. were
prepared from metals and minerals to cure variety
of diseases.
CLASSIFICATION OF GEMSTONES
• Gemstones are classified based on certain
assertive aspects /parameters. viz.,
1.
TYPES
Organic
Imitation
Inorganic
Synthetic
2. Based on Availability
Availability
Precious
Semi-Precious
The precious and semi precious classification of
gemstones is largely based on the Market (Demand
and Supply); it is an arbitrary classification based on
local availability.
3.
OTHER TYPE CLASSIFICATIONS
Chemical
Composition
Optical
Characters
Durability
Crystal
Structure
Physical and Optical properties are the Key elements
to distinguish various types of gemstones
Physical Properties
•
•
•
•
•
•
•
•
•
•
•
Color *
Streak
Habit
Cleavage, Parting Plane,
Fractures.
Lustre (Reflectivity)
Diaphaneity (Transmisivity)
Hardness*
Specific Gravity
Tenacity & Toughness
Chatoyancy
Asterism
Optical Properties
•
•
•
•
•
•
Refractive Index
Dispersion
Pleochroism
Interference Figures
Optic Axis
Optic Sign
COLOUR CAUSING TRANSITION METAL
ELEMENTS IN IDOCHROMATIC GEMS
Chromium
Iron
Manganese
Copper
Green
Green
Red
blue
Pink
orange
Blue
Green
Red
Uvarovite
Peridot
Almandine
lazurite
Rhodocrosite,
Rodonite
Spessartite
Azurite, Turquoise,
Chrysocolk
Malachite, Dioptase
Cuprite
COLOUR CAUSING TRANSITION METAL ELEMENTS
IN ALLOCHROMATIC GEMS
Chromium
Green
Red
Emerald, Gromularite, Cr-tourmaline,
Cr-Chalcedony, Alexandrite
Ruby, Spinal, Topaz, Alexandrite
Chromium
+ Iron
Red
Green
Pyrope
Jadeite, Diopside, Synthetic OPX
Cobalt
Blue
Syn. Spinel, Syn. Quartz & Glass.
Iron
Green
Yellow
Blue
Violet –
blue
Red
Aquamarine, Tourmaline, Green
Sapphire,
Chrysoberyl, Epidote. Yellow sapphire,
Citrine,
Chrysoberyl,
Orthoclase, Soundmen.
Aquamarine, Spinal. Tanzanite, Iolite.
Eudialyte
Iron +
Titanium
Manganese
Blue
Pink
Yellow Green
Violet
Nickel
Green
Vanadium
Green
Blue – Green
to
Purple – red
Blue
Sapphire
Rhodonite,
Rhodocrosite,
Piedmmtite, Syn. OPX.
Andalusite
Tremolite
Syn. Olivine.
V- Emerald
V- Grossularite
V- Sapphire
Syn. CPX
The main parameters of GEMSTONE
• BEAUTY
• RARITY
• DURABILITY
 Gemstones are measured in terms metric carat,
which is equivalent to 200mg (1 carat is equivalent to
0.2 grams). A CARAT is again sub divided into
hundred POINTS (2mg).
 Gem Testing
&
Gemologist
Certificate:
Gemstones are identified based on their properties (P
& O) along with few instruments (Chelsea filter,
Refractrometer, Gemological Microscopes, Hardness
Pencils etc.). The efficiency of a gemologist lies in
identifying a stone without subjecting it to any kind
of destructive tests.
 A certificate from a certified GEMOLOGIST would
certainly yield to its market, through proper
identification and valuation.
Valuation of Gemstones
valuation of gemstones is done through the
4 C’s precept.
 Color
 Clarity
 Cut &
 Carat Weight
EXPLORATION
 Exploration: Refers to geological, geochemical and
geo-physical field work and complimentary laboratory
studies directed to the discovery of workable mineral
concentration.
 Gemstone occurrence is a very rare and a transient
geologic feature; once exhausted
replenished with in a short time.
cannot
be
 Diverse minerals are formed at different geological
environments in different rock types, viz Igneous,
Sedimentary and Metamorphic rocks, also few
organically.
For Example:
 Diamonds in Kimberlites (Ultra-mafic HTHP)
 Beryl in Pegmatites
 Quartz in Felsic rocks
 Peridot in Ultra mafic rocks
 Garnets in Metamorphic Rocks
T h e p rin c ip le r o le o f a n
E x p lo r a t io n G e o lo g is t
G e m Q u a l it y E m e r a ld
C r y s t a l in Q u a r t s a nd
H o st
Roc k
E s s e n t ia l
M in e r a l
A lm a n d in e G a r n e t
e m b e d d e d in
BERYL
 By the name Beryl is a Silicate of Beryllium and
Aluminum; is the principle store of beryllium in the
earth's crust. In its rare gem form it is notable for a
variety of vivid allochromatic color with vivid names.
 Chemical Composition: Be3Al2Si6O18
• Crystal System: Hexagonal
• Habit: Usually well formed hexagonal prisms, either
flattened or elongate, with pinacoidal terminations.
• Hardness: 7.5-8
• Specific Gravity: 2.67-2.78
• Refractive Index: 1.57-1.59
Distinguishing Properties
• Low R.I
• Low S.G*
• Dichroism
• Low Birefringence
Distribution:
 Common rocks that hosts Beryl are Granites and
Pegmatites.
 Vugs in these rocks may consists of fine crystals of
Beryl, Tourmaline, Topaz, Spodumene etc.
 Mineralization occurs, where the Pegmatites intruding
the basis and ultra-basic rocks yield important gem
deposits*.
Varieties of Beryl
Sl No.
Variety
Color
Cause
1
Goshenite
Uncolored
No Specific Cause
2
Heliodor
Yellow
UVCT (O2-- Fe3+)
3
4
Aquamarine
Green Beryl
Blue
Green
5
6
Morganite
Bixbite
Pink
Red
IVCT (Fe2+  Fe3+)
UVCT+IVCT
Mn2+
Mn2+
7
Maxixe
Blue
Color Centre (No3)
8
Maxixe -typed
Blue
9
Emerald
Color Centre (Co3)
Cr3+/ V3+
Green
(Velvety)
Emerald
 Velvety green variety of Beryl
if known as Emerald.
 CC: Be3(Al,Cr)2 Si6O18
 The typical velvety green hue
of emerald is distinctive.
 The green shade of emerald
caused on the account of
chromium/vanadium impurity.
 RI- 1.578-1.584;
 SG- 2.72
Aquamarine
• Palish blue, light blue green
variety of beryl is known as
aquamarine.
• Sky blue color stone is the most
valuable aquamarine.
• They exhibits a greenish blue
tinge due the phenomenon of
pleochroism.
• Golden colored variety of beryl
is known as Golden beryl.
• The yellow color is due to the
presence of Fe3+ in
concentration of over 1%.
a
• Yellow green variety of beryl is
known as Heliodor.
• Yellow green olive color is the
typical character of Heliodor.
• Bixbite is an extremely rare
variety of beryl.
• Bixbite is strong ruby red or
violet red in color.
• Very rarely found, have an
different genesis to that of
In
found in pegmatite.
extrusive magmatic rocks.
• The red color is due to the
presence of Mn2+ is as an
their
crystal
impurity
in
structure.
Case Study: Ndola Rural Emerald
Fields, Zambia
• Nolda –Rural Emerald field of the Zambian Republic
represents a good source of easily mined emeralds.
• To this day, many areas are a preserve of artisinal
miners. Lack of capital has lead to inefficient
exploration and exploitation.
GEOLOGICAL SETUP:
• Zambia lies in between Zaire and the Zambian cratons
characterized geologically as being ancient mobile
belt; comprising of Igneous, Sedimentary and
metamorphic formations of the Precambrian age.
• Mineralization in the Ndola Rural Emerald Fields
occurs where the quartz-tourmaline-pegmatite
veins intrude the talc-chlorite-tremolite-magnetite
schist and alter the contact zone as biotite-phaloghite
rock, this alteration zone forms the chief ore body.
Three distinct varieties of pegmatite are known in the
study area:
1st Generation: initial feeder phase rich in quartz
and feldspar and contain minor fine grained
tourmaline and not related to mineralization.
2nd Generation: veins composed largely of
tourmaline commonly associated with emerald
mineralization.
•
•
•3
rd
Generation: composed of coarse quartz
tourmaline (5-10 mm) always found in association
with mineralization of emerald.
• The distribution of emerald is very erratic; major
emerald mineralization, rich in volume & grade are
associated with flat lying undulating veins and
number of vein intersects.
• Granitic emplacement characterized by intensive
pulses of pegmatite activity rich in minerals B, Be, F
etc intruding through the well developed shear
system appears to be the cause.
• Crystallization of beryl appears to have taken place
during metasomatic reaction between the slow
cooling pegmatite (3rd Generation)
and the
surrounding schist.
The role of a GEOLOGIST in the field of
•
•
•
•
•
Gemological Science:
Prospecting of a gem deposit.
Establishing the properties of gem minerals.
Identification of stone in cut and rough form.
Enhancement of stone (synthesis).
Minerals grow in a definite physico-chemical
environment, thus a geologist is better able
to understand and reason out the paragenesis
of a mineral or gemstone. It is easy to
distinguish a natural stone from its synthetic
counterpart.
Thanks
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