Gemology 101: Principles And Practices

Gemology 101: Principles And Practices

Presented ByScott 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

Before Modern Gemology (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

Before Modern Gemology (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

Gemology Theory

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


Demonstration Of Polarization OfLight In A Doubly Refractive Gem


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


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


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.



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



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



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

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 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


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


The Use Of The Dichroscope To Test For Double

Refraction Should Be Ignored If Colors Are Weak

Refractometers

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)


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


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


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


View Of “50/50” Spot Method


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


Flat Facet R.I. Reading


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


Turning Polarizing Filter Over Instrument Changes R.I. In A Doubly Refractive Gem


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


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


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)


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


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


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


The Refractometer Serves As One Method For

Finding The Optic Character (Uniaxial Or

Biaxial) For Doubly Refractive Gems Using The

Flat Facet Method


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


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



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

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


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


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


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


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


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



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


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


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


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


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


Single “Bow-Tie” Double “Bow Tie”


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


Bulleye's Eye Iron Cross


Left Airy's Spiral Right Airy's Spiral


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

Gemological Microscopes

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


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


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


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


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


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


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


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


Peridot

R.I.: 1.64 - 1.70

Birefringence: .036

Note Facet Doubling

Under Table


It Is Also Possible To Compute The Refractive

Index Of A Gem Using A Microscope (However,

It Is Not As Accurate As A Refractometer)


First, You Measure The Actual Depth Of The

Gem Using A Micrometer (If Not Mounted) Or A

Leverage Gauge (If Mounted And Open backed)


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


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


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

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


Ruby Spectrum (Both Natural And Synthetic)


There Are Two Methods In Using A

Spectroscope Depending On The

Transparency Of The Gem In Question:

Transmission And Reflection


If The Gem Is Transparent To Translucent,

Then You Should Place A White Light Behind

The Gem And View The Gem Through The

Spectroscope


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

Ultraviolet Radiation (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

Ultraviolet Radiation(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

Chelsea Filters

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


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


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


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

Magnetism

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


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 Microscopes

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

Spectrometry

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)


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


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


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


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


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


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


Spectrophotometric Graph Of Emerald And Ruby

X-rays

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


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


Visible Fluorescence To X-rays And Its

Strength Allows A Laboratory Gemologist

To Distinguish Between Saltwater And

Freshwater Pearls


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


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-ray Fluorescence Is Used Primarily To

Distinguish Between Natural And

Synthetic Emeralds And Rubies

Radiation Detectors

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

Putting It All Together(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


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


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


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


Take R.I. Reading(s) And, In Some Cases, Plot

The R.I. Readings To Determine Optic

Character


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


If Possible, Determine The Optic Character

With The Polariscope If The Gem Is Doubly

Refractive


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


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


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


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.)


Congratulations!

You Have Successfully LearnedHow To Identify An Unknown Gem

Questions Or Comments

Scott B Britton, GIA Graduate Gemologist

Please Email Questions Or Comments To

[email protected] Visit Our Web Site Athttps://www.metdia.com

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Gemology 101: Principles And Practices