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.