Gemology 101: Principles And Practices

Gemology 101: Principles And Practices
Presented By
Scott B Britton, GIA Graduate Gemologist
© 2009 Metdia Corporation. All Rights Reserved.

Class Outline
 Brief History Of Modern Gemology
 Basic Gemology Theory
 Primary Gemology Tools
 Secondary Gemology Tools
 External Laboratory Tools
 Putting It All Together
 Hands-On Lab Time

Class Goals
 Teach Basic Gemology Principles
 Give You Experience With Primary
Gemology Tools And Gem Identification
 Familiarize You With External Lab Tools And
When To Take Advantage Of External Labs

What The Class Will Not Cover
 Diamond Grading
 Colored Stone Grading
 Precious Metals Testing
 Any Material Outside The Realm Of Basic Gem
Identification

Disclaimer
Although The Information Contained In This
Presentation Is Derived From GIA Material, This
Class Does Not Substitute For Official GIA
Coursework. This Class Will Introduce You To
Basic Gem Identification, But Will Not Cover All
The Complexities Involved In Making Positive ID's
Of Unknown Gems.

Brief History Of
Modern Gemology

Before Modern Gemology
 Transparent Red Gems Were Considered
Rubies
 Transparent Blue Gems Were Considered
Sapphires
 Transparent Green Gems Were Considered
Emeralds

(Continued …)
There Are A Few Famous Examples Of Gems
Misnamed Because Of This Type Of
Misclassification. For Example, The Large Red
“Ruby” In The British Crown Jewels Is Actually A
Red Spinel

(Continued …)
If Someone Wanted To Examine Or Test A Gem,
The Only Available Methods Were Chemical
And/Or Physical And Would Require Either The
Gem Be Destroyed Or, At Minimum, Need
Recutting And/Or Repolishing (With Weight Loss)

Modern Gemology
 Late Nineteenth Century To Present
 Progression Pushed Forward By The Discovery
Of Diamonds In South Africa
 Goal Was To Develop A Non-Destructive,
Scientific Method Of Classifying Gems

Modern Gemology
(Continued …)
 In 1931 Robert M. Shipley Starts GIA, Whose
Mission Is To Ensure The Public's Trust Of
Diamonds, Colored Stones, And Pearls
Through Education And Research
 Richard T. Liddicoat, Jr., The “Father Of
Modern Gemology,” Develops The Modern GIA
Diamond Grading Scale In 1953

Refraction Is The Principle That A Beam Of Light
Traveling In One Medium Will Bend And Either
Speed Up Or Slow Down As It Enters Another
Medium
Refraction Index

Refraction Index
(Continued …)
The Ratio Of The Velocity Of Light In The Old
Medium To The Velocity Of Light In The New
Medium Is Known As The Refractive Index.

Refractive Index
(Continued …)
Bending Of Light Entering Gem

Refractive Index
(Continued …)
 Measured From The Ratio Of The Velocity Of
Light In Air To The Velocity Of Light Inside A
Gem, Refractive Index Is A Unique “Fingerprint”
For Each Gem
 However, Refractive Index For Each Gem Type
Can Be Broken Down Even Further Depending
On The Crystal Structure Of The Gem

Movement Of Light
 As Light Moves Through Air (Or Any Other
Gas), It Vibrates Perpendicularly In Waves
From The Direction It Was Transmitted.
 When Light Enters Another Medium, Not Only
Does The Velocity Of The Light Change, The
Direction It Is Transmitted Is Affected By The
Crystal Structure Of The New Medium

Movement Of Light
(Continued …)

Single Refraction
 Single Refraction Is When A Gem's Structure
Does Not Change The Vibration Of The Light
Waves Transmitted Other Than Changing The
Velocity, Direction, And/Or Partially Absorbing
The Light Waves' Energy
 Light Waves In A Gem That Has Single
Refraction Vibrate Evenly In All Directions

Single Refraction
(Continued …)
 Gems That Fall Under The Category Of Being
Amorphous (Lacking A Regular Crystal
Structure) Or That Crystallize In The Cubic
Crystal System Are Singly Refractive
 Singly Refractive Gems Have One Refractive
Index Ratio

Double Refraction
 Double Refraction Is A Condition Where One
Light Wave Is Split Into Two Light Waves
Traveling Perpendicular To One Another And
Vibrating At Different Rates As It Enters The
New Medium From Air (Called Polarization)

Double Refraction
(Continued …)
 Doubly Refractive Gems Are Gems That Are
Not Amorphous And Do Not Crystallize
Cubically

Double Refraction
(Continued …)
Demonstration Of Polarization Of
Light In A Doubly Refractive Gem

Double Refraction
(Continued …)
 Gems That Are Doubly Refractive Have
Refractive Indices That Vary Between A
Minimum And Maximum For Each Gem
Depending On The Direction Of Observation
 The Measure Of A Gem's Ability To Convert A
Single Ray Of Light Into Two Unequal Velocity
Waves Is Known As Birefringence

Double Refraction
(Continued …)
 Doubly Refractive Gems Can Be Further
Broken Down Into Two Subcategories: Uniaxial
And Biaxial
 Uniaxial Gems Have One Direction, Called The
Optic Axis, Where They Do Not Polarize Light
 Biaxial Gems Have Two Directions Where They
Do Not Polarize Light

Double Refraction
(Continued …)
 Uniaxial Gems Can Be Positively Or Negatively
Signed
 Uniaxial With Positive Sign Have A Fixed Top
R.I. And A Varying Bottom R.I. Over A 180º
Cycle. In Addition, Every 180º The Varying
Bottom R.I. Equals The Fixed Top R.I.

Double Refraction
(Continued …)

Double Refraction
(Continued …)
Gems That Are Uniaxial With Negative Sign Are
Similar To Their Positive Siblings With The
Exception That The Top R.I. Varies And The
Bottom R.I. Is Fixed Over A 180º Cycle. In
Addition, Every 180º The Varying Top R.I. Equals
The Fixed Bottom R.I.

Double Refraction
(Continued …)
 Biaxial Gems Can Be Positively Signed,
Negatively Signed, Or Without Sign
 Positively Signed Biaxial Gems Are Where
Both R.I.'s Vary, But The Bottom R.I. Varies
More Than The Top R.I Over A 180º Cycle. In
Addition, They Reach A Common
Measurement At Some Point In The 180º Cycle

Double Refraction
(Continued …)
Negatively Signed Biaxial Gems Are Similar To
Their Positively Signed Siblings With The
Exception That The Top R.I. Varies More Than
Bottom R.I. Over A 180º Cycle. In Addition, They
Reach A Common Measurement At Some Point
Over The 180º Cycle

Double Refraction
(Continued …)
Gems That Are Biaxial Without Sign Have Top
And Bottom R.I.'s That Vary Equally Over A 180º
Cycle. In Addition, The Top And Bottom R.I.'s
Reach A Common Measurement At Some Point
Over The 180º Cycle

Primary Gem
Identification Tools

Primary Gem Identification Tools
 Dichroscope (Mid-To-Late Nineteenth Century)
 Refractometer (1885)
 Polariscope (First Developed For Gem Industry
In 1935)
 Gemological Microscope (First Developed For
Gem Industry In 1938)

Dichroscopes
 Dichroscopes Work On The Principle That Light
Waves That Are Perpendicular To One Another
And Vibrating At Different Frequencies Will
Display Different Colors
 Dichroscopes Allow A User To Check A Gem
For Pleochroism, A Condition Where Different
Colors Show From Different Viewing Angles

Dichroscopes
(Continued …)
Dichroscopes Use A Clear Medium With High
Birefringence (Normally Calcite Or Polarized
Polaroid Filters) To Allow A User To View Two
Colors Per Direction In A Doubly Refractive Gem
(If Distinct, And Except In The Direction Of The
Optic Axis)

Dichroscopes
(Continued …)
Colors As Viewed Through Calcite Dichroscope

Dichroscopes
(Continued …)
 Dichroscopes Can Only Be Used With Gems
That Are Transparent To Translucent
 To Use A Dichroscope, You Hold The Unknown
Gem In Front Of A White Light And Then View
The Gem Through The Instrument
 You Must View Gems From At Least Three
Different Directions To Check For Pleochroism

Dichroscopes
(Continued …)
 Gems That Show The Same Two Colors In All
Directions Where Light Is Polarized Are
Dichroic
 Gems That Show Three Colors (But Only Two
At A Time In Each Single Direction Where Light
Is Polarized) Are Trichroic

Dichroscopes
(Continued …)
 The Use Of The Dichroscope To Test For
Double Refraction Should Be Ignored If Colors
Are Weak

Refractometers
 Probably The Best Tool For Gem Identification
 Refractometers Are Used To Measure The
Refractive Index (Indices) Of An Unknown Gem

Refractometers
(Continued …)
 Refractometers Work On The Principle Of
Measuring The Critical Angle (Bending) Of
Light Entering And Exiting The Gem From A
Known Medium (Usually A High Lead Glass
Surface)

Refractometers
(Continued …)
 There Are Two Methods For Taking Refractive
Indices In A Gem: The Spot Method And The
Flat Facet Method
 The Spot Method Is Used To Take R.I.'s When
There Is Not A Flat Facet On A Gem

Refractometers
(Continued …)
 You Will Typically Get Only One R.I. Reading
Using The Spot Method Unless The Gem Has
High Birefringence
 The R.I. Value Obtained From The Spot
Method Will Vary Between The Minimum And
Maximum R.I.'s For A Doubly Refractive Gem
Depending On The Direction Taken

Refractometers
(Continued …)
 The Most Common Method For Reading A
Spot R.I. Is To Read The R.I. Value Where The
Gem Is Half Lit, Half Dark (Called The “50/50”
Method)
 Sometimes The “50/50” Method Does Not Work
And You Must Find The Average Between
Where The Gem Is Lit And Where It Is Dark

Refractometers
(Continued …)
View Of “50/50” Spot Method

Refractometers
(Continued …)
 The Flat Facet Method Requires You To Take
R.I.'s In Seven Different Directions From 0º To
180º (Every 30º) On At Least One Facet Of A
Gem
 You Take A R.I. Reading For A Gem Where The
Colored Or Shadowed Line Ends Using The Flat
Facet Method

Refractometers
(Continued …)
Flat Facet R.I. Reading

Refractometers
(Continued …)
Turning A Polarized Filter 90º Over The
Refractometer's Viewing Lens Checks For The
Presence Of A Second R.I. Using The Flat Facet
Method

Refractometers
(Continued …)
Turning Polarizing Filter Over Instrument
Changes R.I. In A Doubly Refractive Gem

Refractometers
(Continued …)
Singly Refractive Gems Will Show The Same
Refractive Index (+/- Allowance For Each Gem
Species/Variety) For Any Direction That The
Refractive Index Is Taken Using Either The Spot
Method Or The Flat Facet Method

Refractometers
(Continued …)
Depending On The Angle, Direction, And Type Of
Doubly Refractive Gem, Either You Will Get Two
R.I.'s Between A Minimum And Maximum
Threshold (For Directions Where Light Is
Polarized) Or A Single R.I. (For Directions Where
Light Is Unpolarized) For R.I.'s Taken With The
Flat Facet Method

Refractometers
(Continued …)
If You Only Get A Single R.I. After You Have
Taken Measurements 180º On A Single Side Of A
Gem, You Should Take Measurements Again On
At Least Two More Sides Because Doubly
Refractive Gems Do Not Polarize Light In At Least
One Direction (The Optic Axis)

Refractometers
(Continued …)
To Find The Birefringence Of A Doubly Refractive
Gem, You Subtract The Lowest Refractive Index
Recorded On A Single Side Where Light Is
Polarized From The Highest Refractive Index
Recorded On The Same Side

Refractometers
(Continued …)
Although Refractometers Are The Best Tool For
Reading Refractive Indices, They Are Restricted
To Taking Refractive Index Values From 1.30 To
Either 1.81 Or The Refractive Index Of The Fluid
Used To Make The Make The Contact Between
The Refractometer's Hemicylinder And The Gem

Refractometers
(Continued …)
 If You Record A Refractive Index Reading Of
1.81, In Most Cases You Will Need To Rely On
Other Tests To Identify The Unknown Gem
 There Is An Instrument Called A Reflectivity
Meter Which Can Measure R.I.'s Above 1.81,
But It Is Less Reliable Than A Refractometer

Refractometers
(Continued …)
 The Refractometer Serves As One Method For
Finding The Optic Character (Uniaxial Or
Biaxial) For Doubly Refractive Gems Using The
Flat Facet Method

Refractometers
(Continued …)
 Using A Piece Of Graph Paper And The 14
High And Low Refractive Indices, Plot The
Values Horizontally Starting At 0º And Ending
At 180º

Refractometers
(Continued …)
If Your Graph Has An R.I. Line That Is Fixed And
One That Varies Without Meeting At A Common
Measurement Somewhere In The 180º Cycle Or
You Have Two Fixed Lines, Then You Have An
Uniaxial Gem, But You Will Need To Retake The
R.I.'s On A Different Gem Side To Determine Sign

Refractometers
(Continued …)

Refractometers
(Continued …)
If Your Graph Has Two R.I. Lines That Vary But
Do Not Have A Common Measurement In The
180º Cycle, Then You Have A Biaxial Gem, But
You Will Need To Take R.I.'s On A Different Gem
Side To Determine Sign

Polariscopes
Useful For Detecting Whether A Gem Is Singly Or
Doubly Refractive (Or An Aggregate) And The
Optic Character (Uniaxial Or Biaxial) And
Pleochroism Colors (If You Do Not Have Access
To A Dichroscope) In Doubly Refractive Gems

Polariscopes
(Continued …)
Polariscopes Work On The Principle Of Placing A
Transparent To Translucent Gem Between
Crossed, Polarized Analyzing Windows And
Viewing The Reaction Of The Gem As Light Is
Passed Through The Bottom Analyzing Window,
Through The Gem, And Finally Exiting The Top
Analyzing Window To Your Eyes In A 360º Cycle

Polariscopes
(Continued …)
Crossing polarized analyzing windows causes
light being transmitted from the polariscope's
base through the two analyzing windows to be
blocked and the view to be dark

Polariscopes
(Continued …)
If You Place A Gem In Between Crossed
Analyzers And Light Is Not Visible As You Rotate
It 360º In At Least Three Different Directions, Then
The Gem Is Singly Refractive And You Are
Finished With Polariscope Testing

Polariscopes
(Continued …)
If The Gem Appears Visible As You Rotate It 360º
In At Least Three Different Directions Between
Crossed Analyzing Windows As Light Is Being
Transmitted, Then The Gem Is An Aggregate And
You Are Finished With Polariscope Testing

Polariscopes
(Continued …)
 Gems That Alter The Visibility Of Themselves
In The Polariscope Are Either Singly Refractive
Showing Anomalous Double Refraction Or Are
Doubly Refractive
 Gems That Are Singly Refractive Showing
A.D.R. Are The Result Of Irregularities Within
The Crystal Structure

Polariscopes
(Continued …)
Gems That Are Doubly Refractive Display Light
Every 180º Between Crossed Analyzing Windows
Because Light Is Polarized At 90º Angles Within
The Gem's Crystal Structure And Essentially
Cancels The Polarization Of Light Between The
Analyzing Windows

Polariscopes
(Continued …)
To Separate Whether A Gem Is Singly Refractive
Showing A.D.R. Or Doubly Refractive, Turn It To
The Brightest Lit Position And Then View The
Reaction Of The Gem As You Turn The Top
Analyzing Window So That The Two Analyzing
Windows Are No Longer Crossed And Are
Displaying Light

Polariscopes
(Continued …)
 If The Gem Gets Lighter Than It Was Through
Crossed Analyzing Windows, Then It Is Singly
Refractive
 If The Gem Stays The Same Or Gets Darker
Than It Was Through Crossed Analyzing
Windows, Then It Is Doubly Refractive

Polariscopes
(Continued …)
It Must Be Noted That The Polariscope Has One
Significant Limitation. Certain Red Gems With A
Refractive Index Above 1.71 Can And Do Give
False Readings When Viewed With The
Polariscope. Therefore, You Should Rely On
Other Tests To Confirm Single Or Double
Refraction For This Category Of Gems

Polariscopes
(Continued …)
 You Can Determine The Optic Character
(Uniaxial Or Biaxial) Of A Doubly Refractive
Gem Using The Polariscope
 You Turn The Analyzing Windows So That
They Are Crossed And Hold The Gem In
Between The Two Windows, Looking For A
Color Pattern Similar To An Oil Slick On Water

Polariscopes
(Continued …)
 Using A Condensing Sphere And The
Refractometer's Magnifier, You View The
Colors Through The Condensing Sphere And
Note The Appearance
 If The Colors Have A “Bow Tie” Appearance In
The Condensing Sphere, The Gem Is Biaxial

Polariscopes
(Continued …)
Single “Bow-Tie” Double “Bow Tie”

Polariscopes
(Continued …)
 If The Appearance Of The Colors In The
Condensing Sphere Shows A Shape Similar To
An Iron Cross, A Bull's Eye, Or A Pinwheel
(Airy's Spiral), Then The Gem Is Uniaxial
 You May Or May Not Be Able To Resolve The
Optic Character Using The Polariscope

Polariscopes
(Continued …)
Bulleye's Eye Iron Cross

Polariscopes
(Continued …)
Left Airy's Spiral Right Airy's Spiral

Polariscopes
(Continued …)
You Can View Pleochroism Colors In A Doubly
Refractive Gem Through The Polariscope By
Crossing The Analyzing Windows And Viewing
The Gem Every 45º Where The Light Is Visible In
A Direction Where The Gem Polarizes Light

 Good Gemological Microscopes Work On The
Principle Of Magnifying A Gem In Stereo (As If
You Were Looking At The Object Without
Magnification)
 The Best Tool For Observation Of Inclusions,
Blemishes, Separating Synthetics, Simulants,
Or Imitations From Natural Gems, Etc.
Microscopes

Microscopes
(Continued …)
 You Should Begin All Work Sessions First By
Adjusting The Magnification Of Your
Microscope Because Temperature, Movement,
And Atmosphere Affect The Focus Of The
Optics
 You Begin Focusing The Microscope By
Adjusting The Width Of The Oculars

Microscopes
(Continued …)
 You Then Close The Iris Diaphragm To A Small
Hole And Turn On The Microscope's Well Light

Microscopes
(Continued …)
 You Then Take The Focusable Ocular Out,
Turn The Zoom To 10x, And Focus The
Remaining Stationary Ocular With The
Microscope Arm Focus Adjustment On The
Small Lit Hole In The Iris Diaphragm

Microscopes
(Continued …)
 You Then Take Out The Nonfocusable Ocular,
Put In The Other Focusable Ocular, And Then
Adjust That Ocular At The Ocular Base Until
The Small Lit Hole Comes Into Focus
 You Then Put The Other Ocular In And Check
To See If The Focus Is Clear In Stereo. If Not,
You Restart The Process

Microscopes
(Continued …)
To Use A Microscope, You Should Begin And End
With Low Power Zoom (10x), Using Higher
Powers To Focus And Identify Individual
Characteristics That Assist In Identifying An
Unknown Gem

Microscopes
(Continued …)
Although There Are Many Methods One May
Examine A Gem Through A Microscope, The
Method I Use Is To Mentally Break The Gem Into
Eight Pie Slices And Examine The Crown First
Starting From The Noon Position Going
Clockwise And Then Examine The Pavilion
Likewise

Microscopes
(Continued …)
If You Rotate A Gem On Its Horizontal Axis (Top
To Bottom), Then Inclusions That Were Present
On The Top Half Of The Gem When Viewing The
Crown Will Now Be Located In The Bottom Half
Of The Gem When Viewing The Pavilion And Vice
Versa For Inclusions In The Bottom Half

Microscopes
(Continued …)
If You Rotate A Gem On Its Vertical Axis (Left To
Right), Then Inclusions That Were Present On
The Left Half Of The Gem When Viewing The
Crown Will Now Be Located In The Right Half Of
The Gem When Viewing The Pavilion And Vice
Versa For Inclusions In The Right Half

Microscopes
(Continued …)
You Can Also Identify If A Gem Is Doubly
Refractive By Viewing The Facet Junctions
And/Or Inclusions Inside A Gem From At Least
Three Different Angles For The Presence Of
Doubling Due To The Splitting Of A Single Light
Wave Into Two Separate Waves In A Doubly
Refractive Gem

Microscopes
(Continued …)
 Peridot
 R.I.: 1.64 - 1.70
 Birefringence: .036
 Note Facet Doubling
Under Table

Microscopes
(Continued …)
 It Is Also Possible To Compute The Refractive
Index Of A Gem Using A Microscope
(However, It Is Not As Accurate As A
Refractometer)

Microscopes
(Continued …)
 First, You Measure The Actual Depth Of The
Gem Using A Micrometer (If Not Mounted) Or A
Leverage Gauge (If Mounted And Open
backed)

Microscopes
(Continued …)
 You Then Place The Stone In A Microscope,
Making Sure That The Table Of The Stone Is
Parallel To The Microscope's Objective
 You Adjust The Microscope To The Highest
Magnification It Can Achieve (The Higher The
Magnification, The Closer The R.I.) And Then
Focus The Microscope On The Gem's Top

Microscopes
(Continued …)
 You Then Tape A Table Gauge To The
Stationary Side Of The Microscope Arm And
Make A Mark At Zero On The Movable Portion
 You Then Focus The Microscope On The
Bottommost Point Of The Gem And Then Use
A Loop To Read And Record The Change In
Position Of The Mark On The Table Gauge

Microscopes
(Continued …)
To Compute The Approximate Refractive Index
For The Gem, You Divide The Actual Depth
Recorded With The Micrometer Or Leverage
Gauge By The Apparent Depth Recorded From
The Table Gauge On The Microscope's Arm

Secondary Gem
Identification Tools

 Spectroscope
 Ultraviolet Radiation
 Chelsea Color Filter
 Magnets
Secondary Gem ID Tools

Spectroscopes
Spectroscopes Work On The Principle That
Certain Chemicals In Gems Absorb And Reflect
Certain Colors Of The Basic Visible Light
Spectrum Of Approximately 400nm (Blue) To
750nm (Red)

Spectroscopes
(Continued …)
The Chemical Components Of A Gem Show A
Basic Color Absorption Signature (With Minute
Variations Depending On Certain Trace Chemical
Elements Distinct To The Mining Environment)
Across A Color Prism Inside The Spectroscope
Which Can Be Compared With Known Signatures

Spectroscopes
(Continued …)
Ruby Spectrum (Both Natural And Synthetic)

Spectroscopes
(Continued …)
 There Are Two Methods In Using A
Spectroscope Depending On The
Transparency Of The Gem In Question:
Transmission And Reflection

Spectroscopes
(Continued …)
 If The Gem Is Transparent To Translucent,
Then You Should Place A White Light Behind
The Gem And View The Gem Through The
Spectroscope

Spectroscopes
(Continued …)
If The Gem Is Semi-Translucent To Opaque,
Then You Must Attempt To Read The Signature
By Reflecting A White Light Off The Surface Of
The Gem At A 45º Angle Into The Spectroscope

 Ultraviolet Radiation Works On The Principle
That Certain Elements Within A Gem's
Chemical Structure Either Fluoresce And/Or
Phosphoresce In Reaction To UV Light
 The Presence Of Fluorescence And/Or
Phosphorescence In Certain Gems Can Assist
In Determining An Identification
Ultraviolet Radiation

(Continued …)
 You Check For Fluorescence In A Gem Under
The Presence Of Longwave And/Or Shortwave
UV Light In A Darkened Environment
 You Check For Phosphorescence In A Gem By
Checking For The Existence Of Visible Color In
A Gem While In Darkness After The UV Light
Source Is Turned Off

(Continued …)
 Gems Of The Same Species And/Or Variety
May Or May Not React To Ultraviolet Radiation,
So The Test Is Not Solid Proof Of Identity

(Continued …)
 For Example, Myanmar Rubies Typically
Fluoresce Red. However, Thai Rubies
Typically Do Not Fluoresce Because Of The
High Iron Content Within Their Chemical
Makeup

 The Chelsea Filter Works By Allowing Only
Certain Wavelengths Of Yellow-Green and Red
Colors To Be Transmitted Through The Filter
 Gems That Transmit These Colors Will Appear
As Either Green Or Red, While Other Gems
Will Simply Appear Dark
Chelsea Filters

Chelsea Filters
(Continued …)
 Chelsea Filters Are Used To Detect Dyes And
To Indicate The Coloring Agent In Some Gems
And To Separate Some Gem Materials From
Imitations
 However, Certain Synthetic And Natural Gems
Appear The Same Through The Chelsea Filter,
So It Is Only A Supplemental Test

Chelsea Filters
(Continued …)
 For Example, Natural Emerald Turns Red
When Viewed Through The Chelsea Filter;
However, Synthetic Emerald Also Turns Red
When Viewed Through The Chelsea Filter
 In Addition, Demantoid Garnet And Some
Green Zircons Look Pinkish Or Reddish And
Can Be Easily Confused With Emerald

Chelsea Filters
(Continued …)
 There Are Two Methods For Using The
Chelsea Filter: Transmission And Reflection
 If A Gem Is Transparent To Translucent, Then
You Place A White Light Behind The Gem And
View The Gem Through The Chelsea Filter

Chelsea Filters
(Continued …)
If The Gem Is Semi-Translucent To Opaque,
Then You Direct A White Light At The Gem From
A 45º Angle And View The Reflection Through
The Chelsea Filter

 Magnets Are A Useful Tool In Separating Some
Synthetic Diamonds From Natural Diamonds
 Synthetic Diamonds Are Grown Using A
Metallic Flux To Speed Up The Process
 Some Synthetic Diamonds Are Actually
Attracted To Magnets Due To Metallic Flux
Inclusions
Magnetism

Magnetism
(Continued …)
If You Find A Diamond That Is Attracted To A
Magnet, You Should Be Strongly Suspicious That
The Diamond Is Synthetic And Should Further
Test It Using UV Radiation And Looking For
“Hourglass” Or “Stop Sign” Growth Zoning And
Metallic Inclusions Under Magnification

 Electrical Conductometer
 Scanning Electron Microscope
 Spectrometry
 X-rays
 Radiation Detectors
External Laboratory Tools

Electrical Conductometer
Works On The Principle That In Some Diamonds
(Mostly Blue And Gray Type IIb Diamonds),
Electrons In The Gem Are Already Mobile And
Will Conduct Electrical Current Through The Gem
When It Is Applied

Electrical Conductometer
(Continued …)
The Instrument Is Useful For Determining If A
Diamond Is Naturally Blue And Gray Colored Or If
The Diamond Has Possibly Had Its Colored
Enhanced Or Added Due To Irradiation And/Or
Other Man-made Method

Scanning Electron Microscope
 Is 100 Times Or More Powerful Than The
Highest Practical Optical Magnification (200x)
Of The Standard Stereo Gemological
Microscope

Scanning Electron Microscopes
(Continued …)
 Works On The Principle Of Bombarding A Gem
With A Focused Beam Of X-rays And
Electrons, Causing A Secondary Emission Of
Electrons From The Gem's Surface Due To
Displacement

Scanning Electron Microscopes
(Continued …)
 Useful When Coupled With A X-ray Detector To
Determine The Approximate Amounts Of The
Major Chemical Elements Present In A Gem
 Each Chemical Element Emits A Characteristic
Amount Of Energy When Excited By The
Electron Beam, Which The X-ray Detector
Analyzes

 Spectrophotometer (UV Through Visible Light)
 Fluorescence Spectrometer (Fluorescence
Emitted By Gems Exposed To Ultraviolet And
Visible Light Radiation)
Spectrometry

Spectrometry
(Continued …)
 Infrared Spectrometer (Infrared Radiation
Emitted By Gems Exposed To Visible Light
Through The Upper Edge Of The Radio Wave
Spectrum)

Spectrometry
(Continued …)
 Similar In Range And Function To A
Spectroscope, The Spectrophotometer
Analyzes Electromagnetic Energy In Light
 However, The Spectrophotometer Is Much
More Sensitive Than The Human Eye And Can
Detect Energy Present That The Human Eye
Can Not

Spectrometry
(Continued …)
The Spectrophotometer Works By Scanning The
Ultraviolet Through Visible Light Spectrum In Tiny
Nanometer Slices, Recording The Absorption
Pattern Reflected Back To The Instrument From
The Gem, And Displaying It In A Graph With
Peaks And Valleys According To The Light
Absorption

Spectrometry
(Continued …)
Although The Spectrophotometer Can Be Used
On Any Gem Or In Place Of Using A
Spectroscope, The Instrument Is Used Most Often
To Detect Treatment In Diamonds

Spectrometry
(Continued …)
The Fluorescence Spectrometer Is Similar To A
Spectrophotometer With The Exception That It
Measures The Fluorescence Emitted By Gems
Exposed To Ultraviolet And Visible Light
Radiation And Displays It On A Graph

Spectrometry
(Continued …)
Similar To A Fluorescence Spectrometer, The
Infrared Spectrometer Measures A Gem's
Absorption Of Infrared Radiation With The
Exception That Its Range Covers From Visible
Light Through The Upper End Of The Radio
Wave Region

Spectrometry
(Continued …)
Infrared Spectrometry Is Effective In Detecting
Substances Like Epoxy, Stain, Resin, And Plastic,
Which All Show A Characteristic Signature In The
Infrared Range, Without Having To Destroy Or
Mar A Gem To Retrieve A Sample

Spectrometry
(Continued …)
Spectrophotometric Graph Of Emerald And Ruby

X-rays
 Work On The Principle That Due To Their Short
Wavelengths And High Energy, X-rays Can
Penetrate Material That Ordinary And UV Light
Radiation Can Not
 X-Rays Are The Only Method Of Positively
Separating The Different Types Of Pearls

X-rays
(Continued …)
Similar To Taking X-rays On Humans, Pearls
Subjected To X-rays From X-radiography Display
A Characteristic Signature Which A Trained
Laboratory Gemologist Can “Read” To Tell If They
Are Cultured Or Natural

X-rays
(Continued …)
 Similar To UV Fluorescence, X-ray
Fluorescence Tests For The Emission Of
Energy, Visible Or Invisible, That Results From
Exposure To The Higher Energy Wavelength
Of X-rays

X-rays
(Continued …)
 Visible Fluorescence To X-rays And Its
Strength Allows A Laboratory Gemologist
To Distinguish Between Saltwater And
Freshwater Pearls

X-rays
(Continued …)
 Another X-ray Instrument In The Arsenal Of A
Lab Is Energy Dispersive X-ray Fluorescence
 Energy Dispersive X-ray Fluorescence Works
On The Principle That X-ray Radiation Will
Stimulate The Emission Of Other Wavelengths
Which Are Themselves In The X-ray Region Of
The Radio Wave Spectrum

X-rays
(Continued …)
 Energy Dispersive X-ray Fluorescence
Determines A Gem's Chemical Composition By
Analyzing The Characteristic Wavelength And
Amount Of Energy Each Chemical Element
Produces And Compares It To Known
Standards

X-rays
(Continued …)
 X-ray Fluorescence Is Used Primarily To
Distinguish Between Natural And
Synthetic Emeralds And Rubies

 Work On The Principle That Irradiated Gems,
Whether Natural Or Assisted By Man, Will Give
Off A Certain Amount Of Radiation Over A
Period Of Time Known As Half-life
 Radiation Detectors Are Not Geiger Counters;
They Are More Sensitive Than Geiger Counters
Radiation Detectors

Radiation Detectors
(Continued …)
 Most Irradiation Is Performed On Gems To
Achieve Color
 It Is Important To Know When Gems Have
Been Bombarded With Irradiation Not Only For
Health Reasons, But Also Because Their Color
Can Be Altered With Heat From Routine
Repairs Done With A Bench Jeweler's Torch

 The First Thing You Do To Begin The Gem
Identification Process Is To View The Unknown
Gem Without Magnification
 You Should Note Color, Phenomena (Distinct
Visual Effects), Transparency, Clarity,
Fashioning, Luster, And Any Surface Breaks
Putting It All Together

(Continued …)
 You Should Analyze Hue, Tone, And Saturation
Of The Color In The Unknown Gem
 In Transparent Gems, Look For Any Color
Change In Either Or Between Daylight
Equivalent Fluorescent Lighting And
Incandescent Lighting

(Continued …)
 In Transparent Gems, You Should Also Look
For Any Pleochroism, Color Zoning (Distinct
Bands Of Color) And/Or Dispersion (Different
Colors Of Light Exiting The Gem To Your Eyes)
 In Translucent To Opaque Gems, You Should
Look For Color Banding, Mottling (Streaks Or
Patches Of Color), Or Any Distinct Markings

(Continued …)
 You Should View Both The Top And Bottom Of
The Gem Looking For Any Distinct, Unnatural
Coloring Differences That Could Be A Strong
Indication Of An Assembled Stone
 Look For Any Mold Marks Or “Orange Peel”
Effects Which Strongly Hint That The Gem Is
An Imitation

(Continued …)
 Look At The Attention To Detail Left By The
Lapidary. Scratches, Abrasions, And Sloppy
Finishing All Point To Inexpensive Material
 Pay Attention To Surface And/Or Fracture
Luster, Which Can Also Assist In Identifying An
Unknown Gem

(Continued …)
 Under Magnification, Note Inclusions And
Check For The Doubling Of Facet Junctions
Within The Gem Using Darkfield Or Fiber Optic
Lighting
 Using Overhead Lighting, Note Any Blemishes
Or Surface Conditions That Are Not Readily
Visible Without Magnification

(Continued …)
 Take R.I. Reading(s) And, In Some Cases, Plot
The R.I. Readings To Determine Optic
Character

(Continued …)
 If The Gem Is Transparent To Translucent, Use
The Polariscope To Determine If An Unknown
Gem Is Singly Refractive, Doubly Refractive, Or
An Aggregate. Remember, Do Not Attempt To
View Red Stones Over A R.I. Of 1.71

(Continued …)
 If Possible, Determine The Optic Character
With The Polariscope If The Gem Is Doubly
Refractive

(Continued …)
 If The Gem Is Transparent To Translucent,
Note Any Pleochroism Colors Through The
Dichroscope From At Least Three Different
Directions. Remember To Ignore Any Weak
Color Differences

(Continued …)
Armed With All The Previous Information You
Have Gathered, Look In Reliable References For
A Match. Starting With The R.I.(s), Narrow Your
Choices According To The Specifications For
Each Gem Species/Variety. If The Gem Species
Calls For An Additional Test Beyond The Primary
Tests, Perform It And Recompile The Results

(Continued …)
If The Information You Gathered From Primary
And Any Secondary Test(s) Does Not Match The
Information Provided In Reliable References, First
Reperform The Tests And Then Consider You
Have Either An Imitation Or You Have An
Anomaly. If It Is Not An Imitation, Refer The Gem
To GIA For Identification

(Continued …)
If You Are Not 100% Confident That You Have
Positively Identified An Unknown Stone (Such As
In The Case Of Synthetic Vs. Natural Gems),
Refer The Stone To GIA For Positive
Identification. No Wrong Identification Is Worth
The Consequences (Damaged Reputation,
Lawsuits, Etc.)

(Continued …)
Congratulations!
You Have Successfully Learned
How To Identify An Unknown Gem

Questions Or Comments
Scott B Britton, GIA Graduate Gemologist
Please Email Questions
Or Comments To
president@metdia.com
Or Visit Our Web Site At
https://www.metdia.com

Gemology 101: Principles And Practices

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Gemology 101: Principles And Practices