The Polariscope
Using the polariscope to determine optic character (isotropic vs anisotropic), detect anomalous double refraction, and observe interference figures.
Introduction
The polariscope uses two polarising filters (a fixed lower polariser and a rotatable
upper analyser) to determine a gem's optic character and reveal strain, twinning, and
anomalous double refraction (ADR). With the filters at 90° (crossed polars), the field
appears dark; an anisotropic gem placed in this field allows polarised light to pass through
in orientations governed by its crystal system.
Rotating the stone through 360° gives the fundamental result: a gem remaining dark is
isotropic (cubic or amorphous); one blinking light and dark four times per rotation is
doubly refractive. [1] Isotropic stones showing patchy irregular
light (tabby extinction) display anomalous double refraction (ADR) from internal strain;
garnet and synthetic spinel are common examples. [2] A stone
remaining bright throughout is a microcrystalline aggregate such as jadeite or chalcedony.
The polariscope result guides all subsequent instrument choice, making it the natural
first optical test after visual examination. [3]
Components and Setup
A polariscope consists of:
Basic Components
- Polariser: Lower filter, fixed position
- Analyser: Upper filter, can rotate
- Light source: Below the polariser
- Stone platform: Between the two filters
Crossed Polars Position
When the polariser and analyser are at 90° to each other, light cannot
pass through; the field appears dark. This is the "crossed polars" position
used for most tests. [1]
When aligned parallel, maximum light passes through (bright field).
Basic Optic Character Test
Place the gem between crossed polars and rotate it 360°:
| Observation | Interpretation | Examples |
|---|---|---|
| Remains dark during rotation | Isotropic (cubic system or amorphous) | Diamond, spinel, garnet, glass |
| Light and dark every 90° (4× blink) | Anisotropic (crystalline, non-cubic) | Quartz, tourmaline, corundum, beryl |
| Remains light (never dark) | Aggregate or microcrystalline material | Jade (jadeite/nephrite), chalcedony |
| Patchy light/dark (tabby extinction) | ADR or strain birefringence | Strained garnet, synthetic spinel |
Anomalous Double Refraction (ADR)
Some isotropic materials show strain birefringence due to internal stress:
What Is ADR?
Anomalous double refraction appears as irregular light patches that don't follow
the normal 4× blink pattern. The stone may show:
- Tabby extinction: Cross-hatched or patchy pattern
- Roiled effect: Swirling patterns
- Partial extinction: Some areas dark, others light [2]
Gems Showing ADR
| Gem | ADR Character | Notes |
|---|---|---|
| Garnet (especially almandine) | Common, often strong | Due to rapid growth or thermal stress |
| Spinel | Occasional | More common in synthetics |
| Diamond | Can show strain patterns | May indicate Type IIa |
| Glass | Often shows strain | From cooling stresses |
| Synthetic spinel | Very common | Diagnostic feature |
Distinguishing ADR from True DR
True double refraction (anisotropic):
- Regular 4× extinction pattern
- Consistent across the stone
- Shows on refractometer as two readings
Anomalous double refraction:
- Irregular, patchy extinction
- Often localised areas
- Single reading on refractometer
Conoscopic Observation (Interference Figures)
With magnification and proper technique, the polariscope can reveal
interference figures that indicate uniaxial or biaxial character.
Setting Up
- Use a converging light source (add condensing lens)
- Position the stone with optic axis vertical
- View through magnifying lens above analyser
- Look for characteristic interference patterns
Uniaxial Figure
Uniaxial crystals (trigonal, tetragonal, hexagonal) show:
- Cross pattern: Dark cross (isogyres) on coloured rings
- Centred optic axis: Cross stays stationary during rotation
- Off-centre: Cross arms sweep through field
The cross indicates viewing down or near the optic axis. [3]
Biaxial Figure
Biaxial crystals (orthorhombic, monoclinic, triclinic) show:
- Two melatopes: Points where optic axes emerge
- Curved isogyres: Dark bands curve as stone rotates
- 2V angle: Separation indicates biaxial character
Practical Limitations
Obtaining clear interference figures can be difficult:
- Requires specific orientation
- Stone must be relatively clean
- Takes practice to interpret
- Not always achievable in cut stones
Twinning Detection
The polariscope can reveal crystal twinning:
How Twins Appear
Twinned crystals have domains with different optical orientations:
- Lamellar twinning: Parallel striations that blink at different positions
- Contact twins: Sharp boundary between regions
- Penetration twins: Interpenetrating domains
Diagnostic Twinning
| Gem | Twinning Type | Appearance |
|---|---|---|
| Corundum | Polysynthetic | Fine parallel lamellae; rhombohedral |
| Quartz | Brazil/Dauphiné | Sectors may show differently |
| Feldspar | Albite twinning | Parallel striations |
| Calcite | Polysynthetic | Parallel lamellae |
| Spinel | Contact twins | Octahedral habit |
Strain in Gemstones
Maintenance and Care
Polariscope maintenance is straightforward:
- Keep polarising filters clean and scratch-free
- Store in dust-free environment
- Check light source regularly
- Handle filters by edges only
- Avoid exposing filters to extreme heat
References
- ↑ 1. Nassau, K. (2001). The Physics and Chemistry of Color (2 ed.). Wiley. ISBN: 978-0-471-39106-7.
- ↑ 2. Read, P. (2014). Gemmology (3 ed.). Butterworth-Heinemann. ISBN: 978-0-08-050722-4. DOI: 10.4324/9780080507224.
- ↑ 3. Anderson, B. (1990). Gem Testing (10 ed.). Butterworth-Heinemann.