gemmology.dev – The Microscope

Introduction

The gemmological microscope is the primary tool for studying inclusions, growth features,
and signs of treatment, and is the standard instrument for separating natural from synthetic
gems and detecting many enhancements. A binocular stereo microscope with variable magnification
of 10×–45× is the standard configuration; darkfield illumination (light entering from the
sides against a dark background) is used for most inclusion observation, while brightfield
(transmitted) illumination reveals colour zoning and growth patterns.

Magnifications of 30×–40× cover most routine work. At 10× (loupe equivalent), overall
clarity and obvious inclusions are assessed; at 60×, fine surface features and micro-inclusions
become visible. Immersion in a liquid of similar RI to the stone eliminates surface
reflections and greatly improves the visibility of internal features: benzyl benzoate
(RI 1.57) suits quartz and beryl; di-iodomethane (RI 1.74) suits corundum and spinel.
^[1] Synthetic ruby produced by the Verneuil (flame-fusion) method
is identified by curved growth striae and spherical gas bubbles visible at 30×–40×, features
absent from natural ruby.

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. ^[2]

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 {cite:read-2014-gemmology}

Liquid	RI	Best For	Safety Notes
Distilled water	1.33	Basic immersion; safe for all gems	Safe
Liquid paraffin (also almond oil and baby oil)	1.47	General purpose; low-RI materials	Safe; readily available
Clove oil	1.54	General purpose	Pleasant odour; low toxicity
Benzyl benzoate	1.57	Quartz, beryl, feldspar	Low toxicity
1-bromonaphthalene (monobromonaphthalene)	1.66	Tourmaline, spodumene, diopside	Lab use only; avoid skin contact
Di-iodomethane (methylene iodide)	1.74	Corundum, spinel, garnet	Toxic; handle with care; store in dark bottles

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 ^[3]
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 the primary tool 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

The Microscope