Other Testing Tools
Chelsea filter, thermal conductivity probes, specific gravity measurement, and other supporting gemmological instruments.
Introduction
Beyond the core gemmological instruments, several supporting tools provide
valuable identification assistance. These range from quick screening devices
to precise measurement equipment.
Each tool has specific applications where it excels, contributing to the
comprehensive approach needed for accurate gem identification.
The Chelsea Filter
The Chelsea filter transmits only deep red and yellow-green light. It was designed
to distinguish Colombian emerald (appears red) from its simulants.
How It Works
The filter absorbs most wavelengths, transmitting only:
- Deep red (~690nm)
- Yellow-green (~570nm)
Chromium-coloured gems transmit red light and appear pink to red.
Non-chromium green gems absorb red and appear green or grey.
Historical Context
Named after the Chelsea Polytechnic in London where it was developed
in the 1930s. Originally intended to separate Colombian emeralds from
green glass imitations.
Reactions
| Stone | Chelsea Filter Reaction | Notes |
|---|---|---|
| Colombian emerald | Pink to red | Due to Cr content |
| Zambian emerald | May appear greenish | Higher Fe content |
| Synthetic emerald | Strong red | Often high Cr |
| Green glass | Green or grey | No Cr content |
| Green tourmaline | Remains green | No Cr content |
| Demantoid garnet | Greenish | Cr present but weak reaction |
| Blue cobalt glass | Red | Cobalt transmits red |
| Blue synthetic spinel | Red | Cobalt-coloured |
| Aquamarine | Green | No Cr content |
Chelsea Filter Warning
Thermal Conductivity Probes
Thermal probes measure how quickly heat is conducted away from a heated tip
placed on the gem's surface.
How They Work
- A heated probe tip touches the gem surface
- Sensor measures rate of heat dissipation
- Diamond conducts heat extremely rapidly
- Most simulants conduct heat much more slowly
Diamond Testing
Diamond has the highest thermal conductivity of any gemstone:
- Diamond: Extremely high conductivity; meter reads "diamond"
- Moissanite: Also high conductivity (problem!)
- CZ, glass, etc.: Low conductivity; clearly distinguished from diamond
The Moissanite Problem
Synthetic moissanite (silicon carbide) has thermal conductivity close to
diamond, so basic thermal probes cannot distinguish them.
Solution: Dual testers that measure both thermal and electrical
conductivity. Moissanite is electrically conductive; diamond is not
(except rare Type IIb blue diamonds).
Thermal Tester Types
| Type | Measures | Distinguishes | Limitation |
|---|---|---|---|
| Basic thermal | Heat conductivity only | Diamond from CZ, glass, simulants | Cannot separate diamond from moissanite |
| Dual tester | Thermal + electrical | Diamond from all simulants including moissanite | Type IIb diamonds may give false reading |
| Multi-tester | Multiple properties | Broad screening capability | Higher cost; requires training |
Specific Gravity Measurement
Specific gravity (relative density) is measured using hydrostatic weighing,
comparing weight in air to weight suspended in water.
Hydrostatic Method
The classic method using Archimedes' principle:
- Weigh stone in air (W₁)
- Weigh stone suspended in water (W₂)
- Calculate: SG = W₁ / (W₁ - W₂)
Requires a precision balance with suspension setup.
Equipment Required
- Precision balance: 0.01g resolution minimum
- Suspension bridge: Holds stone in water
- Container: Deep enough for complete immersion
- Clean water: Distilled or deionised, room temperature
- Tweezers: For handling small stones
Practical Considerations
- Remove air bubbles from stone surface
- Use room temperature water
- Account for any drilling or inclusions
- Porous stones may absorb water
- Very small stones are difficult to measure accurately
SG Quick Reference
| Gem | SG | Notes |
|---|---|---|
| Diamond | 3.52 | Very consistent |
| Corundum (ruby/sapphire) | 3.99-4.01 | High SG |
| Spinel | 3.58-3.61 | Lower than corundum |
| Emerald | 2.67-2.78 | Varies with inclusions |
| Aquamarine | 2.68-2.74 | Similar to other beryls |
| Tourmaline | 3.01-3.10 | Variable by species |
| Quartz | 2.65 | Very consistent |
| Topaz | 3.50-3.57 | Overlaps diamond |
| Zircon | 3.93-4.73 | Very variable |
| Peridot | 3.27-3.37 | Distinctive range |
| CZ | 5.6-6.0 | Much heavier than diamond |
| Moissanite | 3.21-3.22 | Lighter than diamond |
Heavy Liquids
Heavy liquids allow quick sorting of gems by density:
Common Liquids
| Liquid | SG | Gems That Float |
|---|---|---|
| Methylene iodide | 3.32 | Quartz, feldspar, beryl |
| Bromoform | 2.89 | Opal, amber, most plastics |
| Clerici solution | 4.25 | Most gems except zircon, corundum |
Safety Warning
The 10× Loupe
The hand loupe remains essential for field work and quick examination:
Choosing a Loupe
- 10× magnification: Industry standard (FGA, GIA grading)
- Triplet lens: Corrected for distortion
- Quality optics: Clear, sharp image to edge
- Adequate working distance: ~25mm
Loupe Technique
- Hold loupe close to your eye
- Bring stone up to loupe (not loupe to stone)
- Find optimal distance for focus (~25mm)
- Use good lighting (overhead or from behind you)
- Steady both hands against each other or face
What to Look For
A quick loupe examination reveals:
- Surface condition (scratches, chips, wear)
- Obvious inclusions
- Doubling of back facets (high birefringence)
- Junction of doublet/triplet layers
- Overall cut quality and symmetry
Colour Reference Stones
Master stones and colour references ensure consistent grading:
- Diamond master sets: GIA-graded stones for D-Z scale
- Coloured stone references: For hue comparison
- Munsell colour system: Standardised colour notation
- Gem colour charts: Printed references (approximations only)