Diamond
Diamond type classification, the 4Cs grading system, fancy colours, natural vs lab-grown identification, and diamond verification instruments.
Introduction
Diamond is pure carbon (C) crystallising in the cubic system, the hardest natural
material at Mohs 10 and the only gem composed of a single element. Its exceptionally
high refractive index of approximately 2.417 (singly refractive, isotropic) and
dispersion of 0.044 (the highest of any common transparent gem) produce the
brilliance and fire for which it is valued above all other species. SG is 3.52.
Diamonds are classified by nitrogen content into types: Type Ia (aggregated nitrogen,
~98% of naturals) through Type IIb (boron-bearing, blue, electrically conductive and
very rare). [1] Type classification determines colour origin and
treatment susceptibility: Type IIa brown diamonds can be HPHT-treated to colourless.
[2] The GIA 4Cs system (colour D–Z, clarity FL–I3, cut Excellent–Poor,
and carat weight) is the universal grading standard. Lab-grown diamonds (both HPHT and CVD) are now prevalent; the 45.52 ct Hope Diamond (Type IIb, blue, in the Smithsonian)
and the 3,106 ct rough Cullinan remain the most famous natural specimens. [3]
Diamond Type Classification
Diamonds are classified into types based on nitrogen content and its arrangement
in the crystal lattice. Type affects colour, physical properties, and response
to treatments.
Type System Overview
| Type | Nitrogen Content | Characteristics | Frequency |
|---|---|---|---|
| Type Ia | Aggregated nitrogen (A and B centres) | Cape series yellow tint; most common | ~98% of natural |
| Type IaA | Paired nitrogen (A centres) | Near-colourless to light yellow | Common |
| Type IaB | Clustered nitrogen (B centres) | Less yellow; may show H3/H4 centres | Less common |
| Type Ib | Isolated nitrogen | Intense yellow or brown | ~0.1% natural; common synthetic |
| Type IIa | No nitrogen | Colourless or brown; often finest quality | ~1–2% natural |
| Type IIb | Boron (no nitrogen) | Blue; electrically conductive [1][4] | Very rare |
Type Significance
Understanding diamond type is important because:
- Treatment response: Type IIa brown diamonds can be HPHT-treated to colourless [2]
- Colour origin: Type determines cause of colour
- Value: Type IIa diamonds are often exceptional
- Synthetic identification: Type Ib common in HPHT synthetics
- Scientific interest: Type reveals formation conditions
Testing for Type
Diamond type is determined by:
- Infrared spectroscopy: Nitrogen absorption patterns
- UV-Vis absorption: Colour centres
- Photoluminescence: Characteristic emissions
Standard gemmological tools cannot determine type; laboratory analysis is required.
The 4Cs for Diamond
The GIA 4Cs system (Colour, Clarity, Cut, Carat weight) is the universal standard
for diamond grading. Any gemmologist needs to understand this system.
Colour Grading
D-Z Colour Scale
The GIA colour scale grades diamonds from D (colourless) to Z (light yellow/brown):
- D, E, F: Colourless (most valuable)
- G, H, I, J: Near-colourless
- K, L, M: Faint yellow
- N, O, P, Q, R: Very light yellow
- S, T, U, V, W, X, Y, Z: Light yellow
Beyond Z, stones enter the "fancy colour" category where yellow is desirable.
Colour Grading Conditions
Accurate colour grading requires:
- Master stones: Comparison set of known grades
- Standard lighting: D65 daylight equivalent
- Neutral background: White or grey grading tray
- Controlled viewing: Table-down, pavilion up
- Multiple observations: Check from different angles
Colour Price Impact
| Colour Range | Price Impact | Notes |
|---|---|---|
| D-F | Premium | Colourless; highest value |
| G-H | Good value | Face-up appear colourless |
| I-J | Budget friendly | Slight warmth; good value |
| K+ | Discounted | Visible tint; value option |
Clarity Grading
GIA Clarity Scale
| Grade | Name | Description |
|---|---|---|
| FL | Flawless | No inclusions/blemishes at 10× |
| IF | Internally Flawless | No inclusions; minor blemishes at 10× |
| VVS1, VVS2 | Very Very Slightly Included | Minute inclusions; very difficult to see at 10× |
| VS1, VS2 | Very Slightly Included | Minor inclusions; difficult to see at 10× |
| SI1, SI2 | Slightly Included | Noticeable inclusions at 10×; may be eye-visible |
| I1, I2, I3 | Included | Obvious inclusions; may affect durability/beauty |
Clarity Factors
Five factors determine clarity grade:
- Size: Larger inclusions have more impact
- Number: Multiple inclusions reduce grade
- Position: Central inclusions more visible
- Nature: Type of inclusion (feather vs crystal)
- Relief: Contrast with surrounding diamond
Eye-Clean Concept
Many buyers seek "eye-clean" diamonds:
- No inclusions visible to unaided eye
- Typically SI1 and above
- Depends on stone size (larger = more visible)
- Personal perception varies
Cut Grading
GIA Cut Grade (Round Brilliant)
GIA grades round brilliant cut on five levels:
- Excellent: Maximum brilliance and fire
- Very Good: High performance
- Good: Standard appearance
- Fair: Below standard
- Poor: Significant performance issues
Cut Quality Factors
| Factor | Excellent Range | Effect |
|---|---|---|
| Table % | 54–57% | Balance of brilliance and fire |
| Depth % | 59–62.3% | Light return and face-up size |
| Crown angle | 34–35° | Fire and brilliance balance |
| Pavilion angle | 40.6–41° | Critical for light return |
| Girdle | Thin to slightly thick | Durability and face-up size |
Light Performance
Well-cut diamonds maximise:
- Brilliance: White light return
- Fire: Spectral dispersion
- Scintillation: Sparkle pattern when moved
Poor cuts show:
- Windowing: Light passes through
- Extinction: Dark areas
- Fish-eye: Girdle reflection visible
Carat Weight
Carat is the metric unit for gemstone weight: 1 carat = 0.200 grams = 200 milligrams. [3]
Weight-Size Relationship
Standard round brilliant approximate dimensions by weight:
- 0.25 ct: ~4.1mm diameter
- 0.50 ct: ~5.1mm diameter
- 1.00 ct: ~6.5mm diameter
- 2.00 ct: ~8.2mm diameter
Actual dimensions vary based on cut proportions.
Price Per Carat Jumps
Price per carat increases at threshold weights:
- 0.30, 0.40, 0.50, 0.70, 0.90 ct: Minor jumps
- 1.00 ct: Major threshold (20–40% jump)
- 1.50, 2.00, 3.00 ct: Significant premiums
- 5.00+ ct: Exceptional premiums
A 0.99 ct diamond costs significantly less per carat than a 1.01 ct stone.
Fancy Colour Diamonds
Diamonds with colour beyond Z, or with colours other than yellow/brown, are graded
as fancy colours with different criteria.
Fancy Colour Grading
| Grade | Description |
|---|---|
| Faint | Very light colour presence |
| Very Light | Light colour visible |
| Light | Easily seen colour |
| Fancy Light | Entry to fancy grades |
| Fancy | Moderate saturation |
| Fancy Intense | Strong saturation |
| Fancy Vivid | Maximum saturation (most valuable) |
| Fancy Deep | Dark tone; saturation varies |
| Fancy Dark | Very dark tone |
Colour Causes
| Colour | Cause | Rarity |
|---|---|---|
| Yellow | Nitrogen (isolated) | Common fancy colour |
| Brown | Plastic deformation | Most common |
| Blue | Boron | Very rare |
| Green | Radiation exposure [5] | Rare |
| Pink/Red | Plastic deformation [5] | Very rare; most valuable |
| Orange | Nitrogen + deformation | Rare |
| Black | Graphite inclusions | Relatively common |
| White | Milky inclusions | Uncommon |
Value Hierarchy
Fancy colour value (generally, for equivalent saturation):
- Red: Rarest and most valuable
- Blue: Very rare; high value
- Pink: Extremely desirable
- Green: Rare; valuable
- Orange: Relatively rare
- Yellow: Most common fancy; value varies
- Brown: Most abundant; lowest fancy value
Natural vs Lab-Grown Diamonds
Distinguishing natural from laboratory-grown diamonds is increasingly important
as lab-grown quality and availability increase.
HPHT Diamond Characteristics
High Pressure High Temperature synthetics show:
- Metallic inclusions: Iron/nickel flux catalyst
- Colour zoning: May show cross-shaped zones
- Fluorescence patterns: Different from natural
- Magnetic: Metal inclusions attracted to magnet
- Type: Often Type Ib (isolated nitrogen)
CVD Diamond Characteristics
Chemical Vapour Deposition synthetics show:
- Striated growth: Parallel growth lines
- Brown tint: Often HPHT post-treated
- Unusual fluorescence: Orange or other colours
- Very clean: Few inclusions typically
- Type: Usually Type IIa [1]
Detection Methods
| Method | Detects | Limitation |
|---|---|---|
| DiamondView | Growth patterns, fluorescence | Expensive equipment |
| Photoluminescence | Defect centres | Laboratory required |
| UV-Vis absorption | Colour centres | Laboratory required |
| Infrared spectroscopy | Nitrogen type | Laboratory required |
| Magnetic testing | HPHT metal inclusions | Not all HPHT magnetic |
Diamond Verification Instruments
Screening Devices
Various instruments help identify potential synthetics or simulants:
- Thermal probes: Distinguish diamond from most simulants
- Combined thermal/electrical: Separate diamond from moissanite
- Type screening devices: Identify stones needing further testing
- UV screening devices: Detect unusual fluorescence patterns
Laboratory Instruments
| Instrument | Purpose |
|---|---|
| DiamondView | Fluorescence imaging of growth structures |
| FTIR spectrometer | Diamond type and treatment detection |
| UV-Vis-NIR spectrometer | Colour origin analysis |
| PL spectrometer | Defect centre identification |
| Raman spectrometer | Material verification |
When to Test
Consider laboratory testing for:
- Large, high-value diamonds
- Unusually clean stones
- Unusual fluorescence
- Stones without documentation
- Type II diamonds (more likely treated or synthetic)
- Any stone with atypical features
Famous Diamonds
References
- ↑ 1. Breeding, C.; Shigley, J. (2009). The "Type" Classification System of Diamonds and Its Importance in Gemology. Gems & Gemology, 45(2), 96–111. DOI: 10.5741/gems.45.2.96.
- ↑ 2. Fisher, D.; Spits, R. (2000). Spectroscopic Evidence of GE POL HPHT-Treated Natural Type IIa Diamonds. Gems & Gemology, 36(1), 42–49. DOI: 10.5741/gems.36.1.42.
- ↑ 3. Read, P. (2008). Gemmology (3rd ed.). Butterworth-Heinemann. ISBN: 978-0-7506-6449-3. DOI: 10.4324/9780080507224.
- ↑ 4. Fritsch, E.; Rossman, G. (1988). An Update on Color in Gems. Part 3: Colors Caused By Band Gaps and Physical Phenomena. Gems & Gemology, 24(2), 81–102. DOI: 10.5741/gems.24.2.81.
- ↑ 5. Fritsch, E.; Rossman, G. (1988). An Update on Color in Gems. Part 2: Colors Involving Multiple Atoms and Color Centers. Gems & Gemology, 24(1), 3–15. DOI: 10.5741/gems.24.1.3.