RGM-100 Gem Refractometer — How to Measure Refractive Index and Identify Gemstones

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A gemstone's appearance can be beautiful, but appearance alone is not enough to establish its identity. Natural gemstones, laboratory-grown stones, glass imitations, and other simulants can look remarkably similar. For gemologists, jewelers, jewelry appraisers, and gemstone enthusiasts, measuring refractive index (RI) is one of the most useful methods for narrowing down a gemstone's identity.

The NI Supply RGM-100 Gem Refractometer is a portable optical instrument designed to help users measure gemstone refractive index using a viewing scale, an optical measuring prism, appropriate illumination, and refractive-index contact fluid. Its compact design makes it suitable for gemstone testing in jewelry workshops, gemology classrooms, laboratories, and other settings where portability is important.

This guide explains how a gem refractometer works, how to prepare and measure a gemstone, how to interpret the results, how to investigate double refraction, and how to care for your instrument.

What Is a Gem Refractometer?

A gem refractometer is an optical instrument used to measure the refractive index of a suitable gemstone or gem material.

The refractive index describes how light propagates through a material relative to a vacuum. In practical terms, it describes how light changes direction when it enters a material at an angle, although the underlying optical relationship is more precisely expressed through the speed of light in the material.

Gemologists use refractive index because different gemstone species—and sometimes different varieties within a species—have characteristic RI values. When the measured value is compared with reliable gemological reference data, it can help distinguish between possible identities.

For example, a refractive-index reading can help narrow the possibilities when examining a transparent faceted stone that might be sapphire, spinel, garnet, quartz, or a simulant. The result must be interpreted alongside other evidence, such as color, optical character, inclusions, specific gravity, and other appropriate tests.

Important: A refractometer does not independently prove that a gemstone is natural, untreated, or authentic. Its readings are one part of a broader identification process.

How the RGM-100 Gem Refractometer Works

The RGM-100 uses an optical measuring system to produce a light/dark boundary that can be read against a refractive-index scale.

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The basic measurement process involves four elements:

1. Optical prism: The gemstone is placed against the measuring surface using a suitable contact liquid.

2. Contact fluid: A thin film of refractive-index liquid helps establish optical contact between the gemstone and prism by reducing air gaps.

3. Illumination: Light enters the instrument through its designated light-entry area.

4. Viewing scale: The optical system produces a shadow boundary whose position corresponds to a refractive-index reading.

When the instrument is correctly illuminated and the gemstone is properly positioned, the observer sees a boundary across the scale. The position of this boundary is used to determine the gemstone's refractive index.

The RGM-100 has a stated measuring range of approximately 1.30–1.81 RI, with scale graduations of 0.01 RI. The practical result depends on the stone, the quality of optical contact, illumination, focusing, and the user's ability to read the boundary.

RGM-100 Gem Refractometer: Features and Specifications

The RGM-100 is intended for portable gemological testing and is supplied with accessories to help users get started.

Portable gemological instrument

Compact design for gemstone testing at a workbench, in a classroom, or in a laboratory.

1.800 RI contact fluid included

The listed package includes a 10-gram vial containing approximately 1.8 mL of refractive-index fluid.

789 nm flashlight

A dedicated light source is included to illuminate the instrument as intended by its design.

Specifications and package contents are based on the NI Supply product listing and should be checked against the exact unit and package supplied. The 0.01 scale graduation describes the scale divisions; it should not be interpreted as a guarantee of measurement accuracy of ±0.01 under all conditions.

See the product and included accessories: RGM-100 Gem Refractometer — NI Supply.

Step-by-Step: How to Use the RGM-100 Gem Refractometer

Correct technique is essential for obtaining useful, repeatable readings. Contamination, poor optical contact, an unsuitable gemstone facet, and improper illumination can all make a reading difficult to interpret.

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Step 1: Prepare the instrument

Place the RGM-100 on a stable work surface. Open the upper cover to expose the measuring prism.

Inspect the prism and the gemstone facet you intend to test. Both should be clean and free of dust, grit, and residue that could interfere with optical contact.

Choose a suitable, relatively flat facet. A properly polished facet generally provides better contact than a rough, curved, chipped, or heavily included surface.

Step 2: Apply refractive-index contact fluid

Place a small drop of suitable gem refractometer contact fluid on the measuring prism using an appropriate applicator.

The fluid helps eliminate air gaps between the gemstone and prism, allowing the optical system to produce a readable boundary.

The RGM-100 product package listed by NI Supply includes 1.800 RI contact fluid. This fluid is an important part of the measurement system, not merely a cleaning agent.

Use only a compatible fluid, apply it sparingly, and follow the supplier's safety instructions. Contact liquids can be hazardous or incompatible with certain materials, coatings, and treatments.

Step 3: Position the gemstone

Gently place the selected flat facet against the fluid-covered prism. The fluid should form a thin, even film between the stone and the optical surface.

Avoid pressing hard or sliding the gemstone across the prism. Excessive pressure or abrasive particles can damage the measuring surface.

If the stone does not sit securely or cannot establish good optical contact, choose another suitable facet if possible.

Step 4: Illuminate the instrument

Use the supplied flashlight and direct illumination toward the instrument's designated light-entry window, following the instrument's configuration and instructions.

Adjust the light angle until the scale and boundary are visible. Avoid aiming a bright light directly into your eye or the viewing eyepiece.

The supplied 789 nm flashlight is the specified light source for this package. Do not assume that a different flashlight or wavelength will produce identical readings or viewing characteristics.

Step 5: Focus the eyepiece

Look through the eyepiece and adjust it until the scale markings are sharp.

You are looking for the boundary between a lighter and darker region of the viewing field. Depending on the lighting and optical conditions, the boundary may appear as a distinct edge or a narrow colored line.

A blurred or poorly defined boundary is not a reliable basis for a precise measurement. Recheck the focus, lighting, cleanliness, and gemstone contact before proceeding.

Step 6: Read the refractive index

Locate the point where the shadow boundary crosses the refractive-index scale.

Read the corresponding scale value and record the result. The RGM-100's stated scale graduation is 0.01 RI, so avoid reporting more precision than the instrument supports.

If the boundary falls between graduations, any interpolation should be treated as an estimate rather than a guaranteed measurement.

Step 7: Repeat the measurement

When practical, repeat the reading after carefully repositioning the gemstone. Record whether the readings are consistent or change with orientation.

Differences may result from poor contact or measurement technique, but they can also provide useful information about the stone's optical properties.

Do not force a reading if the boundary remains unclear or falls outside the readable scale.

Understanding Refractive Index Readings

A refractive-index value becomes useful when compared with reliable reference information for the suspected gemstone.

For example, if a stone produces a repeatable RI reading, a gemologist can compare that result with published values for candidate gem species. If the value is inconsistent with a suspected identity, the discrepancy may help rule out that possibility or indicate that further testing is necessary.

However, gemstones do not always produce one simple RI value.

• Isotropic gemstones generally have one refractive index under the relevant measurement conditions.

• Anisotropic gemstones can have different refractive indices depending on the direction of light propagation and polarization.

• Doubly refractive gemstones may show a range of readings rather than one value.

• Poor optical contact can produce weak, blurred, or misleading boundaries.

Temperature, surface condition, inclusions, coatings, illumination, and operator technique can also affect the quality and interpretation of a measurement.

For reliable identification, compare readings with a reputable gemological reference chart and consider other diagnostic properties.

How to Examine Double Refraction

Double refraction, also known as birefringence, is an important optical property of many gemstones. It occurs when an anisotropic material has different refractive indices for different optical vibration directions.

The RGM-100 package includes a polarizing filter to assist with examining changes in the optical response.

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To investigate possible double refraction:

1. Obtain the clearest stable reading you can using the standard measurement procedure.

2. Observe the boundary while carefully rotating the polarizer as permitted by the instrument's design.

3. If appropriate, reposition or rotate the gemstone gently and observe whether the boundary shifts.

4. Record the lowest and highest reliable readings when the instrument and gemstone allow them to be determined.

5. Compare the observed range with trusted reference information for the suspected gemstone.

The difference between the highest and lowest reliable refractive-index readings may help estimate birefringence when the measurement method is appropriate.

Not every gemstone will show two distinct boundaries, and a changing boundary does not automatically establish a particular gemstone identity. Optical behavior must be interpreted in context.

What Is the Difference Between Refractive Index and Birefringence?

These terms are related but describe different properties.

Understanding these distinctions helps users avoid confusing an instrument reading with a complete gemstone identification.

Why the 1.800 RI Contact Fluid Matters

Contact fluid is essential to the conventional gem refractometer measurement process because it helps establish optical contact between the gemstone and measuring prism.

The RGM-100 package discussed here includes fluid with a nominal refractive index of 1.800. This is a widely used type of contact fluid for gemological refractometry.

The fluid's refractive index is particularly relevant when testing stones near the upper end of the instrument's range. A gemstone whose refractive index is higher than the effective contact-fluid and instrument limits may not produce a conventional, readable boundary.

Consequently, a reading that cannot be obtained does not prove that a stone is fake, nor does it automatically establish that the stone lies beyond the instrument's range. The result could also reflect unsuitable contact, illumination, or technique.

For a deeper explanation, see NI Supply's guide to 1.800 versus 1.81 RI fluid in gemstone testing.

Common Problems and Troubleshooting

When readings remain inconsistent, document the result as inconclusive rather than forcing an interpretation.

Cleaning, Maintenance, and Safe Storage

The measuring prism is a precision optical surface. Proper care protects the instrument and helps preserve measurement quality.

• Remove the gemstone gently after testing.

• Clean residual contact fluid using an appropriate, soft, lint-free optical tissue and compatible cleaning method.

• Never scrape the prism or use abrasive materials.

• Avoid unapproved solvents that could damage the instrument's finish or optical components.

• Keep the instrument dry and store it in its protective case when not in use.

• Keep contact fluid tightly sealed and follow its label and safety data sheet.

• Avoid direct skin or eye contact with contact fluid, and keep it away from children.

• Do not dismantle the optical assembly or force the polarizer or other controls.

If the prism becomes scratched, the optical system appears damaged, or the instrument cannot produce a readable boundary under suitable conditions, seek qualified assistance.

Frequently Asked Questions About Gem Refractometers

Can the RGM-100 determine whether a gemstone is real or fake?

The RGM-100 can help narrow down possible gemstone identities by measuring refractive index. However, RI alone cannot conclusively establish whether a gemstone is natural, laboratory-grown, treated, or a simulant. Additional gemological tests may be required.

What is the refractive-index range of the RGM-100?

The listed measuring range is approximately 1.30–1.81 RI, with scale graduations of 0.01 RI. Actual readability depends on the gemstone, contact fluid, optical conditions, and measurement technique.

Does the RGM-100 include refractive-index fluid?

Yes. The listed package includes approximately 1.8 mL of 1.800 RI contact fluid in a 10-gram vial.

Why is refractive-index fluid necessary?

The fluid helps establish optical contact between the gemstone and the prism by reducing air gaps. Without suitable contact, the optical boundary may be unclear or the measurement unreliable.

Can the RGM-100 measure double refraction?

The included polarizing filter can help users investigate changes in readings associated with optical anisotropy. The ability to obtain useful readings depends on the gemstone and measurement conditions. Results should be interpreted using appropriate gemological references.

Can I test any gemstone with a refractometer?

No. Some gemstones cannot be measured reliably because of their shape, surface condition, optical properties, or refractive index. Curved cabochons and rough or heavily included stones may be especially challenging with conventional prism-contact methods.

How accurate is the RGM-100?

The published scale graduation is 0.01 RI. Scale graduation is not the same as guaranteed accuracy. Measurement quality depends on correct technique, optical contact, illumination, focusing, and the instrument's condition.

Is the RGM-100 suitable for beginners?

The RGM-100 is a portable instrument that can be useful for students, hobbyists, jewelers, and gemology professionals learning or performing RI testing. Beginners should practice on known reference stones and consult reliable identification charts before drawing conclusions about unfamiliar gemstones.

Choose the RGM-100 Gem Refractometer from NI Supply

A gem refractometer is a valuable addition to a gemstone testing toolkit. When used correctly, it provides an important optical measurement that can help distinguish between possible gemstone identities and guide further investigation.

The NI Supply RGM-100 Gem Refractometer with 789 nm flashlight and 1.800 RI fluid offers a portable solution for gemstone refractive-index testing, with a stated range of 1.30–1.81 RI and a 0.01 RI scale graduation. The listed package includes contact fluid, a polarizing filter, a monochromatic filter, a protective case, instructions, and a gemstone RI chart.

Whether you are studying gemology, evaluating stones in a jewelry workshop, or building a practical gemstone testing setup, understanding the instrument's proper use and limitations is essential to obtaining meaningful results.

Explore the RGM-100 here: Shop the RGM-100 Gem Refractometer at NI Supply.

NI Supply — Refractometers for Specialized Applications

NI Supply provides refractometers and related measurement products for gemology, industrial applications, laboratories, and other specialized uses. We aim to help customers find appropriate instruments for their testing requirements with clear product information and practical guidance.

Visit NI Supply to explore our refractometer selection or contact us with questions about choosing an instrument for your application.