Refractometer in Gemology: A Simple Guide to R.I. Measurement

Learn how to use a refractometer in gemology for accurate refractive index (R.I.) measurement. Understand requirements, limits, and information you can obtain from flat and curved gemstone surfaces.

Refractometer: How It Works

A refractometer is an optical instrument based on the principle of total internal reflection.
It measures the refractive index (R.I.) of gemstones and other substances.

R.I. measurement is one of the most reliable tests in gemology.
It is quick, non-destructive, and often gives the most useful results.

Requirements for Accurate Measurement

For correct readings, you need:

• Optical contact between stone and prism
• A clean, polished surface
• Contact liquid
• R.I. of the stone within the range of the instrument

The light must always travel from a denser to a rarer medium for accurate results.

Limits of a Refractometer

The upper limit of the instrument depends on:

• The refractive index of the prism
• The refractive index of the contact liquid

Most commercial refractometers have an R.I. limit of 1.81.
Some newer contact liquids have a slightly lower limit of 1.79.

What You Can Learn from a Refractometer

Flat surface readings:

• Single or double refraction (S.R./D.R.)
• Birefringence
• Optic character (uniaxial or biaxial)
• Optic sign (positive or negative)

Curved surface readings (distant vision):

• Mean R.I. only

Why This Matters to You

If you test gemstones, a refractometer can be your most valuable tool.

It gives you clear, repeatable results.
It saves you time compared to other methods.
It helps you avoid mistakes in gemstone identification.

Do you already use a refractometer in your testing?
What challenges have you faced while measuring R.I.?

A refractometer measures refractive index (R.I.) using total internal reflection. It helps gemologists identify gemstones quickly and accurately. This guide explains requirements, limits, and key readings from flat and curved surfaces.