Grading and Valuation
Colour grading (hue, tone, saturation), clarity assessment, cut quality, carat weight, and value factors for coloured gemstones.
Introduction
Grading coloured gemstones is more complex than grading diamonds because there is no
universal standard system. The "4 Cs" (Colour, Clarity, Cut, Carat weight) apply, but
colour is overwhelmingly the most important factor—often accounting for 50-70% of
a coloured stone's value.
Professional grading requires controlled lighting conditions, master stones for
comparison (where available), and extensive experience with each gem species.
Colour Grading
Colour is described using three components: hue, tone, and saturation. Understanding
these components is essential for accurate colour communication and grading.
Hue
Hue is the basic spectral colour (red, orange, yellow, green, blue, violet) or a
combination of adjacent hues. In grading, hue is described by:
- Primary hue: The dominant colour seen
- Secondary hue: Modifying colour (e.g., "slightly purplish red")
- Tertiary modifiers: Subtle additional colours in complex stones
A pure, unmodified hue is generally most desirable for the "big three" gems.
Tone
Tone describes the lightness or darkness of a colour on a scale from colourless
(0) to black (10). Ideal tones vary by species:
- Very light (2-3): Often too pale, lacks presence
- Light (3-4): Acceptable for some species (aquamarine)
- Medium (5-6): Often ideal for most coloured gems
- Medium-dark (6-7): Can be excellent if saturation is maintained
- Dark (7-8): May appear blackish, lose colour visibility
- Very dark (8-9): Generally too dark, colour not visible
Saturation
Saturation describes the intensity or purity of the colour, from grey (low
saturation) to vivid (high saturation). Saturation is the key differentiator
between fine and commercial goods.
- Greyish: Low saturation, washed out
- Slightly greyish: Moderate saturation
- Moderately strong: Good saturation
- Strong: High saturation
- Vivid: Maximum saturation (most valuable)
Ideal Colours by Species
| Gem | Ideal Hue | Ideal Tone | Key Notes |
|---|---|---|---|
| Ruby | Slightly purplish red | Medium to medium-dark (6) | Vivid saturation; 'pigeon blood' |
| Blue sapphire | Violetish blue to blue | Medium to medium-dark (5-6) | Kashmir 'velvety' most prized |
| Emerald | Bluish green to green | Medium (5-6) | Vivid saturation; 'jardin' tolerated |
| Padparadscha | Pink-orange (balanced) | Light to medium (4-5) | Neither pink nor orange dominant |
| Tanzanite | Violetish blue to blue | Medium (5-6) | Strong saturation |
| Paraíba tourmaline | Greenish blue to blue | Light to medium (4-5) | Neon, electric appearance |
| Tsavorite | Green | Medium (5-6) | Vivid, no yellow modifier |
| Spinel | Varies by colour | Medium (5-6) | Pinkish red most valuable |
Colour Grading Conditions
Clarity Grading
Unlike diamonds, coloured gemstones have no universal clarity grading system. The GIA
developed a clarity type system that sets expectations based on how each species
typically forms.
GIA Clarity Types
| Type | Description | Examples |
|---|---|---|
| Type I | Usually eye-clean | Aquamarine, topaz, tanzanite, citrine |
| Type II | Usually included | Ruby, sapphire, garnet, spinel, peridot |
| Type III | Almost always included | Emerald, red tourmaline, alexandrite |
Clarity Terminology
Common clarity descriptors for coloured stones:
- Loupe-clean: No inclusions visible under 10× magnification
- Eye-clean: No inclusions visible to naked eye (most common standard)
- Slightly included: Minor inclusions visible to naked eye
- Moderately included: Noticeable inclusions but acceptable
- Heavily included: Significant inclusions affecting beauty or durability
Clarity Factors
When assessing clarity impact, consider:
- Size: Larger inclusions have more impact
- Number: Many small inclusions can be worse than one large one
- Position: Central inclusions more distracting than peripheral
- Relief: High-contrast inclusions more visible
- Nature: Some inclusions affect durability (fractures, cleavages)
Emerald Clarity Exception
Cut Quality Assessment
Cut quality affects brilliance, colour display, and face-up appearance. Unlike diamonds,
coloured stone cutting prioritises colour retention and window avoidance over strict
proportions.
Proportions
Key proportion factors:
- Crown height: Affects brilliance and colour modification
- Pavilion depth: Critical for light return and colour
- Table size: Affects brilliance vs. colour depth
- Length-to-width ratio: Personal preference; affects face-up size
- Symmetry: Facet alignment and overall balance
Common Cut Problems
| Problem | Cause | Effect |
|---|---|---|
| Windowing | Pavilion too shallow | See-through area in centre |
| Extinction | Pavilion too deep | Dark, lifeless areas |
| Bowtie | Poor symmetry in ovals/marquises | Dark band across stone |
| Fish-eye | Crown too shallow | Ring visible in table |
| Off-centre culet | Asymmetric cutting | Uneven light return |
Native Cut vs Precision Cut
- Native cut: Cut at origin to maximise weight; often has windows/extinction
- Precision cut: Cut to ideal proportions; sacrifices weight for beauty
- Recut potential: Native cuts may be improved by recutting (with weight loss)
Weight Retention
Face-Up Appearance
The most important assessment is how the stone looks face-up (table facing viewer).
This evaluates:
Brilliance
The amount of light returned to the viewer. Well-cut stones show bright flashes
across the face; poorly cut stones have dark or see-through areas.
Colour Distribution
How evenly colour is distributed across the face. Zoning (colour concentrated in
areas) is common but reduces value if visible face-up.
Scintillation
The pattern of light and dark areas when the stone moves. Pleasing scintillation
shows balanced, lively movement; poor scintillation appears static or blotchy.
Carat Weight
Weight is measured in carats (1 carat = 0.200 grams). Price per carat typically
increases at certain weight thresholds.
Weight-Size Relationship
Different specific gravities mean equal weights look different sizes:
- Low SG (opal 2.15): Looks larger for same weight
- Medium SG (quartz 2.65): Standard reference
- High SG (zircon 4.69): Looks smaller for same weight
A 1-carat opal appears larger than a 1-carat diamond; a 1-carat zircon appears
smaller.
Price Jumps
Prices per carat often jump at key thresholds:
- 1 carat: First major threshold
- 2 carats: Significant premium
- 3 carats: Begins "important stone" territory
- 5 carats: Rare size for most gems
- 10+ carats: Exceptional; auction-level
A 0.99 ct stone may cost significantly less than a 1.01 ct stone of equal quality.
Additional Value Factors
Beyond the basic quality factors, several other elements affect coloured gemstone value.
Treatment Status
Origin
Geographic origin affects value through:
- Historical prestige: Kashmir, Burma, Colombia
- Quality association: Certain origins produce consistently finer goods
- Rarity: Some deposits are exhausted or limited
See Origin Determination for detailed premiums.
Certification
Laboratory reports from recognised labs add value by:
- Confirming identity and natural status
- Documenting treatments (or lack thereof)
- Providing origin determination
- Establishing provenance for insurance/resale
Major labs: GIA, Gübelin, SSEF, AGL, GRS
Provenance
Historical ownership can significantly increase value:
- Royal or noble ownership
- Celebrity provenance
- Historical significance
- Previous auction records
Market Considerations
Factors Increasing Value
- Vivid, saturated colour
- Eye-clean clarity
- Premium origin (documented)
- Untreated status
- Well-proportioned cut
- Important size (3+ carats)
- Lab certification
Factors Decreasing Value
- Pale or dark colour
- Visible inclusions affecting beauty
- Unknown or commercial origin
- Heavy treatment
- Poorly proportioned cut
- Small size
- No documentation