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Benefits of Absorption Spectrum Analysis
Each type of gem has a distinct chemical structure that only allows certain wavelengths of light to reflect or pass through it, absorbing the remaining wavelengths. The absorbed wavelengths make up the gem's color. Absorption spectrum analysis measures the wavelengths, identified by color, absorbed by a particular gem. The procedure of identifying a gem's absorption spectrum determines the actual, true color of a gem, since many types of gems have seemingly identical colors when viewed only with the naked eye. For example, red garnet appears similar to red tourmaline, but the absorption spectra of the two gems vary from each other. Moreover, absorption spectra testing helps identify similarly colored gems with the same density and refractive index.
How to Measure Absorption Spectra
A gemologist uses a spectroscope to identify the absorption spectrum of a particular stone. The wavelengths absorbed by the stone appear as vertical black lines inside the absorption spectrum of the spectroscope.
The first thing you must affirm when using a spectroscope involves the light source itself. If lines appear on the spectroscope when viewing the light source alone, that light source will skew the results of the test.
Shine a verified light source directly on the stone and view the stone closely with the spectroscope. Transparent stones reveal their absorption spectrum more easily, as do larger samples of stone. Once tested, mark down the stone's results and compare those results against an absorption spectra chart to determine the stone's identity.
Typical Absorption of Common Gems
The absorption spectrum appears as a colored band of red, orange, yellow, green, blue, indigo and violet, but the wavelengths also have measurements in nanometers. According to Walter Schumann, author of "Gemstones of the World," aquamarine has bands at 537, 456 and 427 nanometers, with the strongest band at 537 nanometers. Ruby has bands at 694, 693, 668, 659, 476, 465, 468 and the 610-500 nanometer range, with the strongest bands at 694, 694, 476, 465 and 468 nanometers. Pink topaz has a single band at 682 nanometers. Turquoise has an average band at 432 nanometers and weak bands at 460 and 422 nanometers.
Variations within Gem Types
Some gems have differences in their absorption spectra that are dependent upon location. For example, according to Schumann, blue sapphire from Australia has strong bands at 471, 460 and 450 nanometers, while blue sapphire from Sri Lanka has one weak band at 450 nanometers. The same gem with an apparently different color may have a similar spectrum, however. Yellow sapphire also has strong bands at 471, 460 and 450 nanometers, like Australian blue sapphire.