Refractive Index Testing

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Overview

The refractive index (RI) is a fundamental optical property that measures how much light bends when entering a gemstone. It is one of the most important diagnostic tools in gemology.

How It Works

A refractometer measures the critical angle of total internal reflection. Light enters the gem and bends at an angle determined by the gem's optical density. The RI reading helps identify gem species.

Equipment Needed

  • Gemological refractometer
  • Refractive index liquid (RI 1.81)
  • Polarizing filter
  • Light source (sodium or white LED)

Procedure

  1. Clean the gem and refractometer hemicylinder
  2. Apply a small drop of RI liquid to the hemicylinder
  3. Place the gem flat on the liquid with the largest facet down
  4. Look through the eyepiece and read where the shadow edge meets the scale
  5. Rotate the gem to check for double refraction (birefringence)

Typical Values / Results

Gemstone Value / Observation
Diamond 2.417
Ruby/Sapphire 1.762-1.770
Emerald 1.577-1.583
Quartz 1.544-1.553
Topaz 1.619-1.627

Limitations

Cannot read gems with RI above 1.81 (the liquid limit). Curved surfaces and small stones are difficult to measure accurately.

Gemstones That Cannot Be Tested

The following gemstones cannot be fully tested with this method:

Exceptional Brilliance (RI > 2.4)

  • Diamond (RI: 2.417-2.419)
  • Sphalerite (RI: 2.37-2.43)

Outstanding Brilliance (RI 1.9-2.4)

  • Blue Zircon (RI: 1.925-2.015)
  • Yellow Zircon (RI: 1.925-2.015)
  • White Zircon (RI: 1.925-2.015)
  • Hyacinth (RI: 1.925-2.015)
  • Zircon (RI: 1.925-2.015)
  • Demantoid (RI: 1.88-1.94)
  • Sphene (RI: 1.885-2.050)

Excellent Brilliance (RI > 1.81)

  • Kyawthuite (RI: 2.16-2.19)
  • Painite (RI: 1.787-1.816)
  • Benitoite (RI: 1.757-1.804)

Partial Range Issues (upper range exceeds 1.81)

  • Rhodochrosite (RI: 1.597-1.817)
  • Bastnasite (RI: 1.717-1.818)
  • Hibonite (RI: 1.79-1.82)

For these gemstones, alternative testing methods such as specific gravity, spectroscopy, or professional lab equipment with extended RI ranges must be used.