The Refractometer

Refractive index measurement using the gemmological refractometer, including faceted and cabochon techniques, birefringence reading, and troubleshooting.

equipment/refractometer refractive-index birefringence identification

Introduction

The refractometer is the most important gemmological instrument. It measures refractive
index (RI) by observing the critical angle of total internal reflection at the
gem-to-glass interface.

Refractive index is a fundamental property that helps identify gem species and can
distinguish between similar-looking materials.

How It Works

Light enters through the gemstone placed on a high-RI glass hemisphere. At angles
greater than the critical angle, light is totally internally reflected, creating
a shadow edge on the scale. This edge corresponds to the gem's RI.

Components

A standard gemmological refractometer includes:

  • High-RI glass hemisphere: Typically RI ~1.86
  • Contact liquid (RI ~1.81): Bridges the gap between gem and glass
  • Scale: Range typically 1.35–1.81
  • Light source: Monochromatic sodium yellow (589nm) for accurate readings
  • Eyepiece: For viewing the scale and shadow edge

Critical Angle Principle

When light travels from a denser to less dense medium, it refracts away from
the normal. At the critical angle, light travels along the interface. Beyond
this angle, total internal reflection occurs.

The position of the shadow edge on the scale directly indicates the critical
angle and therefore the refractive index.

Faceted Stone Technique

For faceted stones, the standard technique provides accurate RI measurements:

Procedure

  1. Place a small drop of contact liquid on the hemisphere
  2. Position the stone table-down on the liquid
  3. Look through the eyepiece at the scale
  4. Rotate the stone to find maximum and minimum RI readings
  5. For uniaxial stones: two readings (omega and epsilon)
  6. For biaxial stones: three readings (alpha, beta, gamma) but only two visible at once

Reading the Scale

  • Shadow edge: Sharp boundary between light and dark regions
  • Single reading: Isotropic material (cubic or amorphous)
  • Two readings: Anisotropic material; difference is birefringence
  • Moving readings: Indicates high birefringence; rotate to find limits

Birefringence Calculation

Birefringence = Higher RI - Lower RI

For example:

Cabochon (Spot Reading) Technique

For curved surfaces, use the spot method:

Procedure

  1. Use minimum contact liquid
  2. Place the curved surface on the hemisphere
  3. Look for a spot of light rather than a shadow edge
  4. The spot's position on the scale indicates approximate RI
  5. Rock the stone gently to centre the spot

Limitations

  • Less accurate than faceted readings (+/- 0.01)
  • Useful for rough assessment of cabochons and beads
  • Cannot measure birefringence accurately
  • Best for isotropic materials or quick screening

Troubleshooting

Common problems and solutions when using the refractometer:

Problem Possible Cause Solution
No reading visible RI above contact liquid limit (>1.81) Use SG or other tests; stone may be diamond, zircon, etc.
Fuzzy edge Birefringent stone, dirty contact, poor polish Clean surface; rotate stone; check polish
Moving readings High birefringence Rotate and record both extreme values
Very faint reading Poor lighting or positioning Improve lighting; reposition stone
Multiple edges Doubly refractive viewing down optic axis Rotate stone to find clearest readings

Contact Liquid Safety

Maintenance

Proper maintenance ensures accurate readings:

Daily Care

  • Clean hemisphere with soft cloth after each use
  • Remove all contact liquid residue
  • Check light source is working properly
  • Store in protective case when not in use

Calibration Check

Periodically verify accuracy using known materials:

If readings are consistently off, the instrument may need professional calibration.

Reference Values

Common Gem RI Values
Gem RI Range Birefringence
Diamond 2.417 None (isotropic)
Ruby/Sapphire 1.762-1.770 0.008
Emerald 1.577-1.583 0.006
Aquamarine 1.577-1.583 0.006
Spinel 1.718 None (isotropic)
Tourmaline 1.620-1.640 0.020
Quartz 1.544-1.553 0.009
Topaz 1.619-1.627 0.008
Peridot 1.654-1.690 0.036
Garnet (various) 1.74-1.89 None (isotropic)

Related Topics

Optical PropertiesThe Polariscope