| name | identify-gemstone |
| description | Identify gemstones using optical properties, physical tests, and inclusion analysis. Covers refractive index, specific gravity, pleochroism, spectroscopy indicators, and common simulant detection. Use when identifying an unknown gemstone, verifying a seller's claim about species identity, distinguishing natural stones from simulants or synthetics, building gemological literacy through structured observation, or identifying rough material before cutting to ensure safe handling.
|
| license | MIT |
| allowed-tools | Read |
| metadata | {"author":"Philipp Thoss","version":"1.0","domain":"lapidary","complexity":"intermediate","language":"natural","tags":"lapidary, gemstones, identification, mineralogy, optics"} |
Identify Gemstone
Identify gemstones using systematic physical and optical property testing, inclusion analysis, and elimination against known species profiles.
When to Use
- You have an unknown gemstone or suspect gemstone and want to identify the species
- You need to verify a seller's claim about a gemstone's identity
- You want to distinguish a natural gemstone from a common simulant or synthetic
- You are building gemological literacy through structured observation and testing
- You need to identify rough material before cutting to ensure safe handling
Inputs
- Required: A gemstone specimen (loose stone preferred; mounted stones limit testing)
- Optional: Refractometer with contact liquid (RI fluid, 1.81 standard)
- Optional: Dichroscope (for pleochroism testing)
- Optional: Chelsea colour filter
- Optional: Specific gravity balance or heavy liquids
- Optional: 10x loupe or gemological microscope
- Optional: UV lamp (long-wave 365nm and short-wave 254nm)
- Optional: Polariscope (for optic character determination)
Procedure
Step 1: Visual Inspection
Examine the specimen with the unaided eye and then under 10x magnification.
Visual Inspection Checklist:
+--------------------+------------------------------------------+
| Observation | Record |
+--------------------+------------------------------------------+
| Colour | Hue (red, blue, green...), saturation |
| | (vivid, moderate, weak), tone |
| | (light, medium, dark) |
+--------------------+------------------------------------------+
| Transparency | Transparent, translucent, opaque |
+--------------------+------------------------------------------+
| Luster | Adamantine, vitreous, waxy, pearly, |
| | silky, resinous |
+--------------------+------------------------------------------+
| Cut style | Faceted, cabochon, carved, rough |
+--------------------+------------------------------------------+
| Estimated size | Approximate dimensions (mm) and weight |
+--------------------+------------------------------------------+
| Surface condition | Scratches, chips, abrasion, wear pattern |
+--------------------+------------------------------------------+
| Phenomena | Star (asterism), cat's eye |
| | (chatoyancy), play of colour, colour |
| | change, adularescence |
+--------------------+------------------------------------------+
- Note the body colour under daylight-equivalent lighting (5500-6500K)
- Check for colour zoning by viewing through the stone from different angles
- Assess transparency and luster — these narrow the candidate list immediately
- Look for optical phenomena (star, cat's eye, play of colour)
- Record any visible inclusions without magnification
Expected: A complete visual profile including colour, transparency, luster, and any phenomena. This alone narrows candidates to a manageable shortlist.
On failure: If lighting is poor (yellowish indoor light), note the limitation. Daylight or daylight-equivalent bulbs are strongly preferred. Incandescent light shifts colour perception and can cause misidentification of colour-change stones.
Step 2: Physical Property Testing
Test measurable physical properties to narrow the identification.
Key Physical Properties:
+--------------------+------------------------------------------+
| Property | Method |
+--------------------+------------------------------------------+
| Hardness (Mohs) | Scratch test against reference minerals |
| | or hardness pencils. CAUTION: Do NOT |
| | scratch faceted gemstones — use other |
| | tests instead for cut stones |
+--------------------+------------------------------------------+
| Specific gravity | Hydrostatic weighing: |
| (SG) | SG = weight in air / (weight in air - |
| | weight in water) |
| | |
| | Common SG values: |
| | Quartz: 2.65 |
| | Beryl: 2.68-2.74 |
| | Tourmaline: 3.02-3.26 |
| | Topaz: 3.53 |
| | Corundum: 3.99-4.01 |
| | Zircon: 4.60-4.73 |
| | CZ: 5.65-5.95 |
+--------------------+------------------------------------------+
| Heft | Does the stone feel heavier or lighter |
| | than expected for its size? |
| | CZ and zircon feel noticeably heavy |
| | Quartz and glass feel average |
+--------------------+------------------------------------------+
- For rough material: test hardness using Mohs scale reference points
- For cut stones: measure specific gravity using hydrostatic method
- Assess heft — experienced handlers can distinguish CZ from diamond by weight alone
- Note any cleavage planes visible on the surface
Expected: Hardness range (for rough) or SG value (for cut stones) that differentiates between candidate species. SG is often the most powerful single diagnostic for cut stones.
On failure: If hydrostatic balance is unavailable, use the heft test as a rough guide. Stones that feel "too heavy for their size" likely have high SG (>3.5). If hardness testing would damage a cut stone, skip to optical tests.
Step 3: Optical Tests
Apply gemological optical instruments for definitive properties.
Optical Property Tests:
+--------------------+------------------------------------------+
| Test | What It Reveals |
+--------------------+------------------------------------------+
| Refractive Index | Measured on refractometer with RI fluid |
| (RI) | Diagnostic for most species: |
| | Quartz: 1.544-1.553 |
| | Beryl: 1.577-1.583 |
| | Tourmaline: 1.624-1.644 |
| | Topaz: 1.609-1.617 |
| | Corundum: 1.762-1.770 |
| | Spinel: 1.718 |
| | Diamond: 2.417 (OTL on refractometer) |
| | CZ: 2.15 (OTL on refractometer) |
+--------------------+------------------------------------------+
| Birefringence | Difference between high and low RI |
| (BR) | Quartz: 0.009 |
| | Corundum: 0.008 |
| | Tourmaline: 0.018-0.020 |
| | Singly refractive: 0 (spinel, garnet, |
| | diamond) |
+--------------------+------------------------------------------+
| Pleochroism | Colour variation with crystal direction |
| (dichroscope) | Strong: tourmaline, tanzanite, iolite |
| | Moderate: corundum, topaz |
| | None: singly refractive stones |
+--------------------+------------------------------------------+
| Optic character | Singly refractive (SR), doubly |
| (polariscope) | refractive (DR), aggregate (AGG) |
+--------------------+------------------------------------------+
| UV fluorescence | Long-wave and short-wave UV response |
| | Diamond: often blue (LWUV) |
| | Ruby: strong red (LWUV) |
| | Emerald: usually inert |
+--------------------+------------------------------------------+
| Chelsea filter | Transmits deep red and yellow-green |
| | Emerald (Cr): appears red/pink |
| | Aquamarine: appears green |
| | Blue synthetic spinel: appears red |
+--------------------+------------------------------------------+
- Measure RI on refractometer — take both high and low readings for birefringence
- Test pleochroism with dichroscope — rotate slowly and note colour changes
- Check optic character on polariscope (SR vs DR vs AGG)
- Test UV fluorescence under both long-wave and short-wave
- Use Chelsea filter if chromium-coloured stones are suspected
Expected: RI value (to 0.001), birefringence, optic character, pleochroism description, and UV response. Combined with Step 2, this identifies most gemstone species definitively.
On failure: If RI is over-the-limit (OTL, >1.81), the stone is likely diamond, CZ, zircon (high-type), or a high-RI synthetic. Use SG and thermal conductivity to differentiate. If no refractometer is available, rely on SG + visual properties + inclusions.
Step 4: Inclusion Analysis
Examine internal features under magnification for species confirmation and natural vs. synthetic determination.
Diagnostic Inclusions by Species:
+------------------+------------------------------------------+
| Species | Characteristic Inclusions |
+------------------+------------------------------------------+
| Diamond | Crystals (garnet, diopside), feathers, |
| | cloud, graining, pinpoints |
+------------------+------------------------------------------+
| Ruby/Sapphire | Silk (rutile needles), fingerprints, |
| | colour zoning (straight angular), |
| | crystal inclusions |
+------------------+------------------------------------------+
| Emerald | Three-phase inclusions (solid + liquid + |
| | gas), jardin (garden-like fractures), |
| | pyrite crystals |
+------------------+------------------------------------------+
| Tourmaline | Growth tubes, liquid-filled fractures |
+------------------+------------------------------------------+
| Quartz/Amethyst | Tiger stripes, phantoms, two-phase |
| | inclusions, negative crystals |
+------------------+------------------------------------------+
Synthetic Indicators:
+------------------+------------------------------------------+
| Synthetic Type | Telltale Inclusions |
+------------------+------------------------------------------+
| Flame fusion | Curved growth lines (striae), |
| (Verneuil) | gas bubbles (spherical) |
+------------------+------------------------------------------+
| Flux grown | Flux fingerprints (wispy veils), |
| | platinum platelets |
+------------------+------------------------------------------+
| Hydrothermal | Chevron or zigzag growth patterns, |
| | seed plate remnant |
+------------------+------------------------------------------+
| Glass simulants | Round gas bubbles, swirl marks, |
| | conchoidal fracture chips |
+------------------+------------------------------------------+
- Examine the stone under darkfield illumination (gemological microscope) or oblique lighting through a 10x loupe
- Look for species-diagnostic inclusions first
- Check for synthetic indicators — curved striae and gas bubbles are definitive for flame-fusion synthetics
- Note the inclusion type, location, and frequency
- Photograph inclusions if possible for records
Expected: Species-confirming inclusion pattern and natural/synthetic determination. Some species are identified more by their inclusions than by optical properties (e.g., emerald's jardin).
On failure: If the stone is eye-clean and no inclusions are visible at 10x, it may be a very clean natural stone or a synthetic. Lack of inclusions raises the synthetic probability — refer to optical and physical tests for confirmation. Laboratory analysis (FTIR, Raman) may be needed.
Step 5: Identification by Elimination
Cross-reference all collected data to reach a final identification.
- Compile the property profile:
- Colour + transparency + luster
- Hardness or SG
- RI + birefringence + optic character
- Pleochroism + UV fluorescence
- Inclusion pattern
- Compare against reference tables for candidate species
- Eliminate species that conflict with any measured property
- If two or more candidates remain, identify the distinguishing test:
- Example: blue topaz vs. aquamarine — SG is definitive (3.53 vs. 2.70)
- State the identification with confidence level:
- Definitive: Multiple properties confirm a single species
- Probable: Properties consistent with one species, but one test missing
- Uncertain: Conflicting data or insufficient testing — laboratory referral recommended
Expected: A final species identification (e.g., "Natural sapphire, blue, heat-treated") with supporting evidence from each test category. Or a clear recommendation for laboratory analysis if field tests are insufficient.
On failure: If the stone cannot be identified with available equipment, document all measured properties and refer to a gemological laboratory. Provide the measured data to the lab — it accelerates their analysis.
Validation
Common Pitfalls
- Trusting colour alone: Colour is the least reliable identification property. Blue stones include sapphire, topaz, aquamarine, tanzanite, iolite, spinel, glass, and CZ. Always confirm with measurable properties
- Skipping SG on mounted stones: Mounted stones limit testing, but you can still check RI, pleochroism, inclusions, and UV. Document the limitation rather than guessing
- Confusing high-RI synthetics with naturals: Flame-fusion rubies and sapphires have identical RI and SG to natural stones. Only inclusions (curved striae vs. straight growth) differentiate them
- Assuming expensive = natural: Commercial jewellery frequently contains treated, synthetic, or simulant stones. Test every stone regardless of provenance claims
- Damaging the specimen: Never hardness-test a faceted gemstone — it will leave visible scratches. Use non-destructive tests (RI, SG, inclusions) for cut stones
Related Skills
cut-gemstone — Identification determines safe cutting parameters and orientation requirements for the speciesappraise-gemstone — Positive identification is the prerequisite for any meaningful valuationmineral-identification — Field mineral identification methodology using physical properties (prospecting domain) shares the systematic elimination approach
