GTLRSCGem Testing LaboratoryGem identification combines physical measurements and optical observations. Different methods provide different clues; their results are interpreted together.
The methods below appear on GTLRSC’s report letterhead. The examination selected for an individual specimen depends on its characteristics and the question being investigated.
A refractometer provides readings related to how light travels through a gemstone. These readings are compared with reference values for known gem materials.
Refractive-index readings help narrow the possible identity of a stone. Where observable, differences between readings can provide further information about its optical behaviour.
Readings are interpreted alongside other properties. Some materials have overlapping values, and a stone’s surface or the instrument’s measurement range may limit what can be recorded.
Specific gravity expresses a material’s density relative to water. For suitable loose stones, hydrostatic weighing compares measurements taken in air and in water.
Gem materials can look alike but differ in density. Specific gravity provides another property to compare with reference information when assessing identity.
The measurement is considered with optical tests and observations. Specimen size, mounting and measurement conditions can affect whether a useful result is obtainable.
A gemological microscope allows closer examination of a stone’s surface and interior. Different lighting conditions help reveal inclusions, growth features, fractures and other characteristics.
Some features provide clues to gemstone identity, natural or synthetic formation, or possible treatments. Their interpretation depends on the type of material and the combination of observations.
An inclusion or surface feature is assessed in context. Microscopy can guide further testing, and its observations are considered with other examination results.
Some gemstones emit visible light when exposed to ultraviolet radiation. The examiner observes features such as the colour, strength and distribution of that response.
Fluorescence behaviour can offer clues to identity and, in some cases, treatment. A weak or absent response is also an observation; no single fluorescence response establishes identity by itself.
A spectroscope separates light into its component colours, allowing an examiner to look for absorption lines or bands associated with a gemstone.
These patterns can provide information relevant to identity and colour. Their usefulness depends on the material, its colour and the quality of the observable spectrum.
Laboratory findings are interpreted together and recorded in the specimen’s report. This page explains common methods; it is not a test result for a particular gemstone.
Educational references: GIA’s guide to coloured-stone analysis, GIA on absorption spectroscopy, and GIA on microscopic inclusions.