Gemstone Treatments
Heat treatment, filling, diffusion, coating, and detection methods for treated gemstones.
Introduction
Most coloured gemstones on the market today have undergone some form of treatment
to enhance their appearance. Understanding treatments is essential for accurate
identification and proper disclosure.
Heat Treatment
The most common treatment, heat treatment permanently alters colour and clarity
by modifying the chemical state of chromophores or dissolving silk inclusions.
| Gemstone | Effect | Temperature Range |
|---|---|---|
| Blue sapphire | Improves blue colour, dissolves silk | 1400-1800°C |
| Ruby | Improves red colour, dissolves silk | 1200-1800°C |
| Tanzanite | Changes brown to blue-violet | 450-650°C |
| Aquamarine | Removes green tint (yellow component) | 400-450°C |
| Citrine | Heat-treated amethyst | 450-500°C |
| Zircon | Creates blue colour from brown | 900-1000°C |
Detection Signs
- Dissolved or partially dissolved silk (dotted lines)
- Stress fractures around solid inclusions
- Altered/melted inclusions
- "Halo" fractures around zircon crystals
- Colour concentrations along fractures
- Altered absorption spectrum
Fracture Filling
Surface-reaching fractures are filled with glass, oil, or resin to improve
apparent clarity. This treatment requires special care in handling.
| Filler Type | Gemstone | Stability |
|---|---|---|
| Oil (cedar oil) | Emerald | Can dry out over time |
| Opticon/resin | Emerald | More stable than oil |
| Lead glass | Ruby, Sapphire | Damaged by heat, acids |
| Polymer resins | Various | Variable |
Detection Signs
- Flash effect (orange/blue flashes in fractures)
- Gas bubbles in filler
- Flow structures
- Different RI in filled areas
- Blue glow under UV (some resins)
Lead Glass-Filled Rubies
Diffusion Treatment
Elements are diffused into the stone at high temperatures to alter colour.
This can be shallow (surface) or deep (lattice diffusion).
| Type | Element | Result |
|---|---|---|
| Surface diffusion | Titanium (Ti) | Blue colour in sapphire (shallow) |
| Lattice diffusion | Beryllium (Be) | Orange/yellow/pink sapphire (deep) |
| Lattice diffusion | Cobalt (Co) | Blue glass-filled stones |
Detection Methods
- Surface diffusion: Immersion reveals colour concentration along facet edges and girdle
- Beryllium diffusion: LIBS or LA-ICP-MS analysis required for definitive detection
- Colour distribution patterns may indicate treatment
Coating and Surface Treatments
| Treatment | Description | Detection |
|---|---|---|
| Thin-film coating | Metallic oxide layer for colour | Wear marks, different colour in scratches |
| Backing | Foil or paint behind stone | Visible in mounted stones, immersion |
| Waxing | Surface wax for lustre | Hot needle test, solvent test |
| Dyeing | Colour added to porous stones | Colour concentrations, swab test |
Irradiation
Exposure to radiation (gamma rays, electrons, neutrons) can create or modify colour.
| Gemstone | Starting Colour | Result |
|---|---|---|
| Blue topaz | Colourless | Blue (followed by heat) |
| Fancy diamond | Various | Green, blue, yellow, pink |
| Kunzite | Pink | Deeper pink (may fade) |
| Smoky quartz | Colourless | Brown/smoky |
Treatment Acceptance
Widely Accepted
- Heat treatment (corundum)
- Oil in emerald (cedar oil)
- Waxing (turquoise, jade)
Accepted with Disclosure
- Resin filling
- Beryllium diffusion
- Irradiation
Controversial
- Lead glass filling
- Surface diffusion
- Coatings
HPHT Diamond Treatment
High Pressure High Temperature (HPHT) treatment can permanently alter diamond colour.
Originally developed for industrial applications, it's now used to improve gem-quality
diamonds.
Type IIa Diamonds
Type IIa diamonds (nitrogen-free) can be decolourised by HPHT treatment:
- Starting material: Brown Type IIa diamonds
- Result: Near-colourless (D-F colour possible)
- Mechanism: Removes plastic deformation that causes brown colour
- Stability: Permanent; colour won't revert
Fancy Colour Creation
| Starting Type | HPHT Result | Mechanism |
|---|---|---|
| Type Ia (cape series) | Intense yellow or green-yellow | Modifies nitrogen centres |
| Type IaB | Yellow or green | Creates H3/H4 centres |
| Type Ib | Orange or brownish-orange | Modifies isolated nitrogen |
| Type IIa (brown) | Colourless or pink | Removes deformation |
| Type IIb | Enhanced blue | Possible but uncommon |
Detection Methods
HPHT-treated diamonds can be identified by:
- Photoluminescence spectroscopy: Characteristic centres
- Infrared spectroscopy: Modified nitrogen aggregation
- DiamondView imaging: Unusual fluorescence patterns
- Graining patterns: May show altered internal structure
- Laboratory testing required: Cannot be detected with standard tools
Cobalt Diffusion
Cobalt can be diffused into gems during high-temperature treatment, most commonly
in lead glass-filled rubies where it creates artificial blue colour in the glass.
Cobalt in Glass-Filled Stones
Some lead glass fillings contain cobalt to add blue colour:
- Creates bluish tinge that masks brown/orange body colours
- Concentrated in fractures and surface areas
- Can be mistaken for better quality material
- Flash effect may show blue rather than typical orange
Detection
- Blue flash effect: Unusual blue colour in fractures under magnification
- Chelsea filter: Strong red reaction from cobalt
- Spectroscopy: Cobalt absorption bands (around 550nm)
- Chemical analysis: LIBS or LA-ICP-MS confirms cobalt presence
Treatment Stability Chart
| Treatment | Stability | Sensitive To | Care Notes |
|---|---|---|---|
| Heat (corundum) | Permanent | N/A | No special care needed |
| Heat (zircon) | Stable | Extreme heat may affect | Avoid jeweller's torch |
| Oil (emerald) | Temporary | Heat, solvents, time | Re-oil periodically; avoid ultrasonic |
| Resin (emerald) | Semi-permanent | Heat, strong solvents | Avoid ultrasonic and steam |
| Lead glass (ruby) | Fragile | Heat, acids, ultrasonic | Extreme care required |
| Beryllium diffusion | Permanent | N/A | No special care needed |
| Surface diffusion | Shallow only | Repolishing removes | Avoid recutting |
| Irradiation (topaz) | Usually stable | Some may fade | Stable once processed |
| Coating | Fragile | Abrasion, solvents | Will wear off over time |
| HPHT (diamond) | Permanent | N/A | No special care needed |
Treatment Stability Warning
Laboratory Report Terminology
Understanding laboratory report terminology is essential for interpreting treatment
status and communicating accurately.
Treatment Codes
| Code | Meaning | Details |
|---|---|---|
| N or NTE | No treatment evidence | No indication of any treatment |
| H | Heat treatment | Evidence of heating detected |
| H(a) | Heat with foreign residue | Borax or similar flux present |
| H(b) or H(Be) | Beryllium diffusion | Lattice diffusion treatment |
| O(minor) | Minor oil | Light oiling; typical for emerald |
| O(moderate) | Moderate oil | Moderate enhancement |
| O(significant) | Significant oil | Heavy treatment |
| R | Resin | Polymer-filled fractures |
| F | Filled | Fracture or cavity filling |
| C | Coated | Surface coating present |
Interpreting Reports
Key phrases on laboratory reports:
- "No indication of heat treatment": Stone appears unheated
- "Evidence of heat treatment": Heating detected
- "Filler detected": Fracture filling present
- "Clarity enhanced": Treatment improved apparent clarity
- "Colour possibly enhanced": Colour may not be natural
- "Origin undeterminable": Insufficient evidence for origin
Trade Organisation Standards
Industry organisations establish treatment disclosure standards.
CIBJO Blue Books
CIBJO (World Jewellery Confederation) publishes standardised terminology:
- Natural: Formed in nature without human intervention
- Treated: Natural material altered beyond cutting/polishing
- Synthetic: Man-made, same properties as natural
- Imitation: Any material resembling another
Full disclosure of treatments is required under CIBJO standards.
AGTA Enhancement Codes
| Code | Enhancement |
|---|---|
| N | Not enhanced |
| H | Heating |
| O | Oiling/resin infusion |
| F | Filling |
| D | Dyeing |
| B | Bleaching |
| C | Coating |
| U | Diffusion |
| R | Irradiation |
| ASBL | Assembled (composite) |
Disclosure Chain
Treatment information must pass through the supply chain:
- Treater → dealer (must disclose)
- Dealer → retailer (must disclose)
- Retailer → consumer (must disclose)
Failure at any point constitutes fraud or misrepresentation in most jurisdictions.