General Identification of Gems

Gem identification is basically a process of elimination. Gemstones of similar color undergo non-destructive optical testing until there is only one possible identity. Any single test is indicative, only. For example, the specific gravity of ruby is 4.00, glass is 3.15-4.20, and cubic zirconia is 5.6-5.9. So, one can easily tell the difference between cubic zirconia and the other two; however, there is overlap between ruby and glass.

And, as with all naturally occurring material(s), no two gems are identical. The geological environment they are created in influences the overall process so that although the basics can be identified the presence of chemical "impurities" and substitutions along with structural imperfections vary thus creating "individuals".

Identification by Refractive Index

One test to determine the gem's identity is to measure the refraction of light in the gem. Every material has a critical angle, at which point light is reflected back internally. This can be measured and thus used to determine the gem's identity. Typically, this is measured using a refractometer although it is possible to measure it using a microscope.

Identification by Specific Gravity

Specific gravity, also known as relative density, is different depending upon the chemical composition and crystal structure type. Heavy liquids with a known specific gravity are used to test loose gemstones.

Specific Gravity is measured by comparing the weight of the gem in air with the weight of the gem suspended in water.

Identification by Spectroscopy

This method uses a similar principle to how a prism works to separate white light into its component colors. A gemological spectroscope is employed to analyze the selective absorption of light in the gem material. Essentially, when light passes from one medium to another, it bends. Blue light bends more than red light. Depending on the gem material, it will adjust how much this light bends. Coloring agents or chromophores show bands in the spectroscope and indicate which element is responsible for the gems color. The wavelengths displayed are measured in nanometers.

Identification by Magnification