Advanced Optical Properties

1. Place gem between crossed polars (dark position)
2. Insert conoscope lens (or use high magnification)
3. View the interference figure in the gem
4. Rotate gem to find clearest figure (viewing down optic axis)

Best results require:

• Clean, well-polished stone
• Reasonably transparent material
• Appropriate viewing direction relative to optic axis

Uniaxial crystals (hexagonal, trigonal, tetragonal) have one optic axis.
When viewed down the optic axis, they show the "bull's eye" pattern:

• Concentric isochromatic rings: Circles of colour
• Black cross (isogyres): Arms parallel to polariser directions
• Melatope: Central dark point (exit point of optic axis)

As the stone rotates off the optic axis, the cross moves outward but
remains visible in the field of view.

Biaxial crystals (orthorhombic, monoclinic, triclinic) have two optic axes.
Viewed down the acute bisectrix, they show:

• Two melatopes: Emerging points of optic axes
• Curved isogyres: Dark bands that change shape with rotation
• 2V angle: Angle between optic axes (diagnostic)

When rotating the stone 45°:

• Small 2V: Isogyres stay close, barely separate
• Large 2V: Isogyres separate widely, may leave field of view

Uniaxial positive (+): ε > ω (extraordinary ray has higher RI)
Uniaxial negative (-): ε < ω (extraordinary ray has lower RI)

Using a compensator plate:

1. Obtain centred uniaxial figure
2. Insert compensator (e.g., gypsum plate) at 45°
3. Observe colour changes in quadrants:
   • Positive: Blue in NE/SW quadrants, yellow in NW/SE
   • Negative: Yellow in NE/SW quadrants, blue in NW/SE

Biaxial positive (+): β closer to α (lower RI)
Biaxial negative (-): β closer to γ (higher RI)

Alternatively: positive = acute bisectrix is Bxa; negative = acute bisectrix is Bxo

Using compensator plate:

1. Obtain biaxial figure with visible isogyres
2. Insert compensator at 45°
3. Observe colour changes along isogyres

Chromium produces characteristic spectra in many gems:

• 694nm doublet: Sharp lines in red (ruby, red spinel, emerald)
• Red fluorescence: Often accompanies Cr absorption
• Broad bands in yellow-green: General absorption creating red colour

Cr-coloured gems: ruby, red spinel, emerald, alexandrite, chrome tourmaline

Iron produces varied spectra depending on oxidation state:

• Fe²⁺: Bands in blue-green region
• Fe³⁺: Sharp lines, bands in violet-blue
• Fe²⁺/Fe³⁺ charge transfer: Broad absorption (blue sapphire)

Iron is the most common colouring agent and appears in many gems.

Luminescence depends on trace elements:

• Activators: Cause luminescence (Cr³⁺ → red; Mn²⁺ → orange)
• Quenchers: Suppress luminescence (Fe → often quenches)
• Sensitisers: Enhance other activators' effects

Example: Burmese ruby fluoresces strongly (low iron); Thai ruby
fluoresces weakly (high iron quenches).

• Growth strain: Irregular growth creates internal stress
• Thermal stress: Cooling creates tension
• Radiation damage: Creates local strain fields
• Inclusion pressure: Inclusions create surrounding strain

Under crossed polars, ADR appears as:

• Tabby extinction: Irregular light/dark patches
• Strain birefringence: Patchy, doesn't follow crystal directions
• "Tatami" pattern: Cross-hatched appearance (some garnets)

Unlike true birefringence, ADR doesn't show normal 4× blink pattern.

• Similar RI values: Need optic sign to distinguish
• Possible treatment: Spectroscopy may reveal heating signs
• Origin determination: Spectral features may be origin-related
• Natural vs synthetic: Fluorescence patterns differ
• Unusual specimens: May require full optical characterisation

For complete optical characterisation:

1. RI and birefringence: Refractometer
2. Optic character: Polariscope (isotropic/uniaxial/biaxial)
3. Optic sign: Conoscope with compensator
4. Pleochroism: Dichroscope
5. Absorption spectrum: Spectroscope
6. Fluorescence: UV lamp (LWUV and SWUV)

Document all results before reaching identification conclusion.