Step 3: Practical Gemology
Use these gemology cheat sheets to review color and clarity grading, as well as special information on diamonds and opals. You can also review basic gem identification testing procedures for the refractometer, polariscope, and specific gravity.
Each section contains links to International Gem Society (IGS) articles that discuss these topics in greater detail.
Table of Contents
- Evaluating Colored Gemstone Clarity
- Evaluating Gemstone Color
- Diamond Cut Grading
- Opal Terminology
- Refractometer Procedures
- Polariscope Procedures
- Specific Gravity Formula
Evaluating Colored Gemstone Clarity
The Gemological Institute of America (GIA) has devised a clarity grading system that, first, groups colored gemstones into three types:
GIA Clarity Types
Usually eye clean
Almost always included
Chrysoberyl, yellow and green
Quartz, amethyst, citrine, ametrine
Touramline, all but green, red and watermellon
Zircon, all but blue and colorless
Tourmaline, red and watermelon
GIA clarity grades have different meanings, depending on the gem type.
|Clarity||Type I||Type II||Type III|
|VVS||Minute inclusions, difficult to see under 10X. Eye clean.||Minor inclusions, somewhat easy to see with 10X. Usually eye clean.||Noticeable inclusions under 10X. Usually eye clean.|
|VS||Minor inclusions, somewhat easy to see with 10X. Usually eye clean.||Noticeable inclusions under 10X. May be eye visible.||Obvious inclusions with 10X. May be eye visible.|
|SI1||Easily noticeable with 10X. Slightly visible to the unaided eye. Usually low relief.||Obvious inclusions, large or numerous under 10X. Apparent to unaided eye.||Prominent to unaided eye.|
|SI2||Easily visible to the unaided eye. Usually low relief.||Obvious inclusions, large or numerous under 10X. Very apparent to unaided eye.||Very prominent to unaided eye.|
|I1||Moderate effect on appearance or durability.||Moderate effect on appearance or durability.||Moderate effect on appearance or durability.|
|I2||Severe effect on appearance or durability.||Severe effect on appearance or durability.||Severe effect on appearance or durability.|
|I3||Severe effect on both appearance and durability.||Severe effect on both appearance and durability.||Severe effect on both appearance and durability.|
|Dcl||Not transparent||Not transparent||Not transparent|
- 10X: Ten-power magnification, such as that of a hand loupe.
- Eye Clean: Without flaws evident to the naked eye.
- Minute: Something only a trained grader can discern through careful observation.
- Minor: Something a trained grader can see without great effort, but without a significant visual impact.
- Noticeable: Something anyone might notice, with some effect on appearance.
- Obvious: Something immediately visible.
- Prominent: Something that demands your attention and strongly affects the gem’s beauty.
For more information, read the IGS guide to clarity grading colored stones.
To learn more about inclusions, read the IGS four-part series of articles, beginning with Identifying Inclusions Found in Natural Gems.
To learn how to evaluate diamond clarity, read the IGS guide to clarity grading diamonds.
Evaluating Gemstone Color
When describing hue, list modifying terms first and capitalized the predominant color.
|V = Violet
R = Red
O = Orange
Y = Yellow
G = Green
B = Blue
P = Purple
sl = slightly
st = strong
1 Extremely Light
2 Very Light
4 Medium Light
6 Medium Dark
8 Very Dark
9 Extremely Dark
2 Slightly Grayish
3 Very Slightly Grayish
4 Moderately Strong
2 Slightly Brownish
3 Very Slightly Brownish
4 Moderately Strong
For more information on grading gemstone color, read the IGS article, Evaluating Gem Color: Hue, Tone, and Saturation.
To learn about the sources of gem color, read the IGS article on allochromatic and idiochromatic gems.
For information on special problems involving gem color measurement and the development of various color-order systems, read Gemstone Color Measurements and Specifications.
Diamond Cut Grading
Modern diamond cut grading requires very accurate measurements. In the table below, you’ll find the tolerances for measuring a diamond’s dimensions.
Modern Diamond Cut Grading Tolerances
|Crown angle average||0.5°|
|Pavilion angle average||0.2°|
|Average star length||5.0%|
|Average lower-half length||5.0%|
|Average crown height||0.5%|
|Average pavilion depth||0.5%|
Evaluating opal requires a distinct system, unlike those used for diamonds and colored gems. This system uses unique terminology.
Opal Body Tone
An opal’s body tone is the base color of the stone, without taking the play of color into account. All body tones from black to white are assigned a value from N1 to N9.
Types of Natural Opal
The designation “natural opal” means the stone has received no treatments or enhancements, other than cutting and polishing. Such gems fall into three categories or types.
Natural Opal Type 1
An opal that has a substantially uniform chemical composition.
Natural Opal Type 2
Commonly known as boulder opal, this type is naturally attached to the host rock in which it was formed. The opal and the host rock have different chemical compositions.
Natural Opal Type 3
Commonly known as matrix opal, this type is found in pores, holes, or between grains of the host rock in which it formed.
Testing for Birefringence
If the band moves, you have a doubly refractive gem.
If the band doesn’t move, give the stone a quarter turn and test it again. Repeat this step as well. If all your readings remain the same, you either have a singly refractive stone or one with very low birefringence. To make that determination, you must use a polariscope. (See below).
To find the highest and lowest refractive indices (RIs), rotate your gem 30° at a time, for a total of six positions. In each position, test the gem by rotating the polarizing filter. Record the highest and lowest readings at each position.
Next, return the stone to the position where you got the highest reading. Adjust the polarizing filter to where only the highest reading is visible. Now, without moving anything else, give the gem a slight twist to the right, then left. When you find the absolute maximum reading for the gem, record it. Then, repeat this procedure to find the lowest RI.
Subtract the lowest RI from the highest. The difference is the gem’s birefringence.
The five optic characters are: uniaxial positive, uniaxial negative, biaxial positive, biaxial negative, and without sign.
You can sometimes determine optic character with a polariscope. However, for some gems, you can only determine it with a refractometer. Using the readings you took while testing for double refraction, you can also ascertain a gem’s optic character.
Review your readings and interpret the results as follows.
If both RIs remain the same in each position or one RI remains the same and the other varies, you have a uniaxial gem.
While measuring birefringence, you should have found a position with only one RI. If you didn’t, you’ll have to test the stone again on another facet.
Your gem is uniaxial positive if the higher RI is the one that changes. If the lower RI varies, you have an uniaxial negative gem.
If both RIs vary, you have a biaxial gem.
Your gem is biaxial positive if the high RI varies more than the low RI. If the lower value varies the most, your gem is biaxial negative.
Gems Without Sign
If both RIs vary the same amount, you have a gem said to be without sign.
The three-part IGS guide to using the refractometer contains detailed instructions for testing refractive index, taking readings for cabochon-cut gems, and determining optic character and recognizing various optic effects.
Testing for Refractive Index
If the Stone Stays Dark
If your gem stays dark, this indicates one of two possibilities. You might have a singly refractive gem. On the other hand, you could be looking straight down the optic axis of a doubly refractive gem. To make sure that’s not the case, test the gem two more times. Turn it on its side and rotate it 360° as you did before. Then repeat the test.
If the gem stays dark in all positions, you have a singly refractive gem. Otherwise, move on to the next test to eliminate the possibility of anomalous double refractive.
If the Stone Changes from Light to Dark
If the light in your gem changes from light to dark, this indicates one of two possibilities. You might have a doubly refractive gem. On the other hand, you might be seeing anomalous double refraction (ADR) from a singly refractive stone.
To test for ADR, set your top filter to its darkest position. Place the stone so maximum light passes through it. Now, quickly rotate the top filter. If the gem gets lighter, you have a singly refractive gem and were seeing ADR. However, if it stays the same or gets darker, you have a doubly refractive gem.
If you think you’re testing garnet or spinel, verify your doubly refractive results with a dichroscope. These singly refractive gems often show ADR.
If the Stone Stays Light
Identifying a gem’s optic figure will tell you whether it’s uniaxial or biaxial.
The three-part IGS guide to using the polariscope contains detailed instructions for testing gemstone refraction, identifying optic figure and sign, and testing for twinning, dichroism, and crystal strain.
Specific Gravity Formula
The five-part IGS guide to specific gravity testing includes a definition of specific gravity for gemologists. In addition, it contains instructions for various testing methods, including using a balance scale, an electronic scale, and heavy liquids. You can also learn to refine your specific gravity testing techniques.