The Dichroscope
Using the dichroscope to observe pleochroism in coloured gemstones, distinguishing dichroic from trichroic materials.
equipment/dichroscope pleochroism colour identification
Introduction
The dichroscope reveals pleochroism by showing two polarised images side by side.
It's essential for identifying coloured anisotropic gems and can help distinguish
natural from synthetic materials.
Pleochroism—the property of showing different colours in different crystallographic
directions—is diagnostic for many coloured gemstones.
Types of Dichroscope
Two main types are available:
Calcite Dichroscope
Uses a calcite rhomb (Iceland spar) to split the image:
- More accurate: Clear separation of colours
- Traditional choice: Preferred by professionals
- Higher cost: More expensive than polaroid type
- Fragile: Calcite can cleave if dropped
Polaroid Dichroscope
Uses polarising film to create two windows:
- More affordable: Lower cost option
- Durable: Less fragile than calcite
- Good for field use: Compact and rugged
- Slightly less clear: Colours may be slightly muted
Usage Technique
Proper technique is essential for accurate results:
Basic Procedure
- View the gem through the dichroscope with strong transmitted light
- Rotate the dichroscope (not the gem) while observing
- Look for colour differences between the two images
- Rotate the gem to find the direction of maximum colour difference
- Record both colours seen
Tips for Best Results
- Use a strong, white light source
- View through the body of the stone, not the surface
- Check multiple directions (rotate the gem 90°)
- Compare colours side by side in the two windows
- Work with good eye relief (don't press against eyepiece)
Interpreting Results
| Observation | Interpretation | Example Gems |
|---|---|---|
| Two identical colours | Isotropic or viewing down optic axis | Spinel, garnet, glass |
| Two different colours (dichroism) | Uniaxial gem (trigonal, tetragonal, hexagonal) | Ruby, sapphire, tourmaline |
| Three colours visible (trichroism) | Biaxial gem (orthorhombic, monoclinic, triclinic) | Tanzanite, alexandrite, iolite |
| No colour visible | Colourless or very pale stone | Colourless topaz, pale beryl |
Pleochroism Strength
Pleochroism varies from very weak to very strong:
Strength Scale
- None: Isotropic materials (no pleochroism)
- Weak: Slight colour variation, difficult to see
- Distinct: Clear difference but moderate
- Strong: Obvious colour difference
- Very strong: Dramatic colour change between directions
Factors Affecting Visibility
- Colour saturation: Pale stones show less pleochroism
- Viewing direction: Maximum difference perpendicular to optic axis
- Light quality: Strong, white light shows colours best
- Stone size: Larger stones show colours more clearly
Diagnostic Pleochroism
Characteristic Pleochroism by Gemstone
| Gemstone | System | Strength | Colours by Direction | Notes |
|---|---|---|---|---|
| Ruby | Trigonal | Strong | O-ray: purplish red / E-ray: orange-red | Best seen in thick stones |
| Blue sapphire | Trigonal | Strong | O-ray: deep blue / E-ray: greenish-blue | Varies with saturation |
| Emerald | Hexagonal | Distinct | O-ray: blue-green / E-ray: yellow-green | More visible in darker stones |
| Tanzanite | Orthorhombic | Very strong | α: blue / β: violet / γ: purple-red | Trichroic — heat treated shows 2 colours |
| Alexandrite | Orthorhombic | Very strong | α: green / β: orange / γ: red-purple | Colour change + strong pleochroism |
| Andalusite | Orthorhombic | Very strong | α: yellow-green / β: brown-green / γ: red-brown | Distinctive multicolour effect |
| Iolite | Orthorhombic | Very strong | α: violet-blue / β: blue / γ: pale yellow | Water sapphire — dramatic colours |
| Kunzite | Monoclinic | Distinct | α: colourless / β: pink / γ: violet | Fades in light over time |
| Tourmaline (green) | Trigonal | Strong | O-ray: dark green / E-ray: light green | Best viewed down c-axis |
| Tourmaline (pink) | Trigonal | Strong | O-ray: dark pink / E-ray: light pink | Orientation important for cutting |
| Aquamarine | Hexagonal | Weak | O-ray: blue / E-ray: colourless-pale blue | Often not visible |
| Peridot | Orthorhombic | Weak | α: colourless-pale green / β: yellow-green / γ: green | Subtle colour difference |
| Hiddenite | Monoclinic | Distinct | Yellow-green / blue-green | Spodumene variety |
| Zircon (blue) | Tetragonal | Weak | O-ray: blue / E-ray: colourless | Difficult to detect |
| Morganite | Hexagonal | Weak | O-ray: pink / E-ray: pale pink | Requires good stone |
| Topaz (blue) | Orthorhombic | Weak | α: colourless / β: pale blue / γ: pale blue | Difficult to detect |
Crystal Axis Terminology
Try the Interactive Tool
Dichroism vs Trichroism
Understanding the distinction:
Dichroic Gems (Uniaxial)
Trichroic Gems (Biaxial)
Biaxial crystals have three principal optical directions and can
show three distinct pleochroic colours. These belong to:
- Orthorhombic system (topaz, peridot, tanzanite)
- Monoclinic system (kunzite, diopside)
- Triclinic system (kyanite)
To see all three colours, rotate the gem to view along different axes.
Practical Applications
Common Mistakes
Avoid these errors when using the dichroscope:
- Rotating the gem instead of the dichroscope initially
- Using reflected light instead of transmitted light
- Not checking multiple directions in the stone
- Confusing colour zoning with pleochroism
- Testing opaque or heavily included stones (no light transmission)