How do gemologists identify a gemstone? They combine observations, measurements, and reference data. A stone's color offers a starting point, but a reliable conclusion requires evidence that fits together.
This introductory lesson explains the main testing tools, a practical way to organize an examination, and the records needed for a useful gemstone knowledge database.
1. Begin with observation
Record the stone's apparent color, transparency, luster, shape, surface condition, and visible zoning. Examine existing chips or fractures without creating new damage. These observations help select the next tests; they do not establish identity by themselves.
Joel E. Arem emphasizes that color can be a weak diagnostic property and that variations in mineral composition affect measured properties. Reference ranges need careful interpretation rather than a search for one perfectly matching number. [1, pp. 12, 14]
2. Understand the main testing instruments
Gemological microscope
Magnification reveals inclusions, small fractures, surface features, and internal structures. Darkfield illumination helps make internal features visible. Arem gives microscopy a central place in identification because inclusions can carry valuable diagnostic information. An observed feature still needs interpretation in the context of the material being examined. [1, p. 11]
Refractometer
A refractometer measures refractive index, an optical property related to the speed of light in a material. Appropriate measurements can also help determine birefringence. Some gems exceed the measurement range of a standard instrument, and some surfaces or materials do not yield a sharp reading. Record these limits rather than forcing a numerical result. [1, pp. 11, 16–18]
Specific gravity measurement
Specific gravity supplies another line of evidence. Comparing it with optical measurements can narrow the possibilities. Arem describes its importance alongside refractive index. Contemporary GIA practice also includes hydrostatic measurement where appropriate, using a loose stone's weight in air and suspended in water. [1, p. 11; 2]
Polariscope
A polariscope examines a stone's behavior in polarized light and helps assess optical character. It complements refractometer observations. Record optical behavior as an observation before using it to support a proposed identity. [1, pp. 11, 16–17]
Dichroscope
A dichroscope helps examine pleochroism: differences in color associated with different optical directions. Arem discusses pleochroism as a useful optical characteristic, while GIA includes the dichroscope among standard colored-stone testing tools. [1, p. 18; 3]
Spectroscope
A spectroscope separates visible light into its component colors, allowing examination of absorption lines and bands. Some materials show characteristic patterns useful in identification. Record the pattern actually observed, including difficulty seeing it. [1, pp. 11, 18]
3. Separate identity, origin, and treatment
Identifying a material and determining how it formed answer different questions. Arem notes that natural and synthetic counterparts can have overlapping physical properties, making internal characteristics particularly important. A matching refractive index alone does not resolve natural versus laboratory-grown origin. [1, Preface, p. vii]
Treatment status is another question. Modern laboratories may combine microscopy with Raman, infrared, or other spectroscopic methods for specific identification and treatment problems. The appropriate combination depends on the stone and the question under investigation. [3]
4. Follow a documented testing sequence
Proposed learning workflow: This is an educational synthesis, not a decision tree reproduced from Arem.
- Assign a specimen ID. Keep photographs and observations attached to that identifier.
- Describe the specimen. Record appearance, condition, and whether it is loose or mounted.
- Examine it under magnification. Document visible features before selecting further tests.
- Select suitable measurements. Use optical tests and specific gravity where the specimen and equipment allow meaningful results.
- Compare several properties. List plausible materials and identify agreements or contradictions.
- Investigate unresolved questions. Seek additional evidence or laboratory examination when needed.
- Report the conclusion and its limits. Separate observations from interpretations and leave unsupported questions unresolved.
Protect the specimen throughout examination. Arem cautions that hardness testing can damage gems; scratch testing is unsuitable as a routine approach to polished stones. He also discusses X-ray diffraction as a specialist method requiring appropriate facilities and care. [1, pp. 12, 14–15]
5. Build records that can be checked
A useful record lets another reader trace a conclusion back to evidence. The following is a proposed structure for study or future database development:
- Specimen: unique ID, description, condition, and photographs.
- Method: test name, instrument, settings, and measurement limits.
- Observation: result as recorded, including units and uncertainty where applicable.
- Interpretation: candidate identity, supporting evidence, conflicting evidence, and next test.
- Reference: author, title, edition, page, and relevant table or figure.
- Review status: preliminary, checked, or requiring further examination.
Keep reference values separate from measurements on an individual stone. Use explicit entries such as “not measured” or “inconclusive” when appropriate. A missing result must never become a numerical zero.
Questions for further study
Why can different materials share a similar appearance? Which independent measurements could distinguish them? What further evidence would be needed before making a statement about natural origin or treatment? These questions encourage testing explanations against evidence.
Sources and scope
- Joel E. Arem, Color Encyclopedia of Gemstones, Second Edition. Van Nostrand Reinhold, 1987. ISBN 0-442-20833-2. Relevant sections: Preface p. vii; “Identification of Gemstones,” pp. 10–12; and “Sources of Data Used in Text,” pp. 14–18. These are printed book page numbers.
- Nicole J. Ahline and Jessa Rizzo, “Analysis of Gemstones at GIA Laboratories,” Gems & Gemology, Winter 2024.
- Phoebe Shang, “How GIA Analyzes Colored Stones from Start to Finish,” GIA, September 30, 2025.
This original overview uses selected book sections, with modern laboratory context attributed to GIA. It is not a complete extraction of the encyclopedia or a specimen test report. The workflow and record structure are editorial proposals.