The Microscope

Using the gemmological microscope for inclusion study, illumination techniques, and immersion methods to identify gems and detect treatments.

equipment/microscope identification/inclusions magnification illumination

Introduction

The gemmological microscope is essential for studying inclusions, growth features,
and signs of treatment. Magnifications from 10× to 60× are typically used, with
various illumination techniques revealing different internal features.

Microscopy skills are fundamental to separating natural from synthetic gems and
detecting many treatments.

Microscope Types

Several microscope configurations are used in gemmology:

Stereo Microscope

The standard gemmological microscope:

  • Binocular viewing: Comfortable, natural depth perception
  • Variable magnification: Usually 10× to 45×
  • Working distance: Adequate space for manipulation
  • Various illumination options: Darkfield, brightfield, fibre-optic

Compound Microscope

Higher magnification for detailed work:

  • Higher power: 100× to 400×
  • Thinner depth of field: More difficult to use
  • Specific applications: Examining very fine inclusions
  • Not standard for routine gemmology

Magnification Ranges

Different magnifications serve different purposes:

Magnification Application Notes
10× Standard loupe magnification; overview examination FGA/GIA grading standard
20× General inclusion observation Good balance of magnification and field
30-40× Detailed inclusion study Most routine microscopy
50-60× Fine details, surface features Reduced field of view
100×+ Specific investigations Rarely needed; difficult to use

Illumination Techniques

Different lighting reveals different features:

Technique Description Best For
Darkfield Light from sides; inclusions bright against dark background Most inclusions, three-phase, silk
Brightfield Light from below; transmitted through gem Colour zoning, growth patterns, fingerprints
Fibre-optic Directed beam for pinpoint illumination Individual inclusions, surface features
Overhead Reflected light from above Surface features, coatings, wear
Polarised Light through crossed polars Strain, twinning, birefringence

Darkfield Illumination

The most commonly used technique:

How It Works

Light enters from the sides rather than directly below:

  • Background appears dark
  • Inclusions scatter light and appear bright
  • Excellent contrast for most internal features
  • Standard setup for routine examination

What It Reveals

  • Crystal inclusions (bright points or shapes)
  • Silk and needle inclusions
  • Gas bubbles
  • Fractures and feathers
  • Fingerprints (healed fractures)
  • Three-phase inclusions

Brightfield and Other Techniques

Alternative illumination for specific observations:

Brightfield

Light transmitted directly through the stone:

  • Colour zoning: Visible as bands or areas of different colour
  • Growth patterns: Hexagonal zoning in corundum
  • Colour concentration: Around fractures or surfaces
  • Best for: Transparent stones with colour variations

Fibre-Optic Illumination

Directed light for specific features:

  • Pinpoint individual inclusions
  • Trace fractures and feathers
  • Examine drill holes
  • Light difficult-to-reach areas

Oblique and Shadow Illumination

Light at steep angles:

  • Enhances surface texture
  • Reveals polish quality
  • Shows surface-reaching features
  • Highlights laser drill holes

Immersion Technique

Immersing the gem in a liquid of similar RI reduces surface reflections and reveals
internal features more clearly.

Common Immersion Liquids

Liquid RI Best For Safety Notes
Water 1.33 Basic immersion; safe for all gems Safe
Benzyl benzoate 1.57 Quartz, beryl, feldspar Low toxicity
Methylene iodide 1.74 Corundum, spinel, garnet Toxic; handle with care
Clove oil 1.54 General purpose Pleasant odour; low toxicity

Immersion Observations

Immersion reveals:

  • Colour zoning: Much clearer than in air
  • Growth patterns: Curved (synthetic) vs angular (natural)
  • Surface treatments: Diffusion layers
  • Internal features: Without surface reflections

Immersion Warning

Diagnostic Inclusions

Certain inclusions are diagnostic for species or origin:

By Gem Species

Gem Characteristic Inclusions
Corundum Silk (rutile needles), fingerprints, hexagonal zoning
Emerald Three-phase inclusions (Colombian), jardín, calcite
Spinel Octahedral crystals, fingerprints, often cleaner than corundum
Tourmaline Growth tubes, needles, colour zoning
Peridot Lily pads (stress discs around chromite)
Topaz Two-phase inclusions, growth tubes

Natural vs Synthetic

Key differences to look for:

  • Natural: Irregular growth, natural crystal inclusions
  • Flame fusion: Gas bubbles, curved striae
  • Flux synthetic: Flux inclusions, platinum crystals
  • Hydrothermal: Seed plate, chevron patterns

Treatment Detection

The microscope is essential for treatment detection:

  • Heat treatment: Altered silk, stress fractures, melted inclusions
  • Fracture filling: Flash effect, gas bubbles in filler
  • Lead glass filling: Flash effect, trapped bubbles
  • Beryllium diffusion: Colour concentration at facet junctions
  • Laser drilling: Thin tubes reaching inclusions
  • Coating: Iridescence, colour concentration at surface

Related Topics

Gemstone Treatments