The Refractometer

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

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.

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

• 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 = Higher RI - Lower RI

For example:

• Quartz: 1.553 - 1.544 = 0.009
• Tourmaline: 1.640 - 1.620 = 0.020
• Zircon (high): 1.984 - 1.931 = 0.053

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

• 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

Pattern I – A single constant shadow edge confirms the material is isotropic.
Ensure the reading stays truly constant during a full rotation.

Pattern II – Two constant, parallel shadow edges that never move. This occurs when
the c-axis is perpendicular to the table facet. The minimum and maximum RI and
birefringence can be determined, but the optic sign cannot without testing on
an alternative facet.

Pattern III – One constant and one variable edge that join together at one point
during rotation. This confirms uniaxial character with the c-axis parallel to the
table. The optic sign is determinable:

• If omega < epsilon (constant < variable): uniaxial positive
• If omega > epsilon (constant > variable): uniaxial negative

Pattern IV – One constant edge, one variable edge, not touching. This ambiguous
pattern can occur in both uniaxial (oblique c-axis) and biaxial stones.

Pattern V – Shadow edges intersect (cross each other). This confirms biaxial
character. The edges may both converge towards beta.

Pattern VI – Both edges move but do not touch. Confirms biaxial, but the optic
sign cannot be determined without additional information.

Pattern VII – Both edges move and touch at one point. Confirms biaxial with
determinable optic sign.

Uniaxial optic sign (Pattern III):

• omega < epsilon → positive
• omega > epsilon → negative

Biaxial optic sign (when both edges move):

• Observe which shadow edge crosses the point halfway between the maximum
  (gamma) and minimum (alpha) values – this edge gives beta
• Beta closer to alpha → biaxial positive
• Beta closer to gamma → biaxial negative

• 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

Periodically verify accuracy using known materials:

• Quartz: 1.544-1.553
• Fluorite: 1.434 (isotropic)
• Spinel: 1.718 (isotropic)

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