Diffraction grating VS prism

September 18, 2015
A diffraction grating is a

Many gemologists have difficulty mastering the direct-vision spectroscope. This article eases the pain.

Gem testing laboratories make use of a variety of instruments, both high-brow and low. Of primary importance are the microscope, refractometer, ultraviolet light, specific gravity equipment, and the spectroscope. Among these, the direct-vision spectroscope is generally the most difficult to initially master. This fear factor leads students into a self-fulfilling cul-de-sac where lack of skill leads to lack of success and the eventual conclusion that the spectroscope is of little use. Nothing could be further from the truth. The spectroscope is one of the most powerful instruments in a gemologist’s quiver. But, like any weapon, you hit little if you depend on luck alone. You gotta practice.

The tremendous value of the spectroscope as an analytical tool lies in its ability to save time in gem testing, by providing initial identification of many gems, with the microscope being the only other tool required.

For example, let’s say you are given a dark blue transparent stone set in a closed back mounting. The construction of the mounting does not permit the refractometer to be used.OPL diffraction grating spectroscope With the microscope and fiber-optic illumination you locate a small well-formed transparent crystal in the gem, and using the fiber-optic light with the spectroscope you observe absorption lines in the blue region at 450, 460 and 470 nanometers (nm), a typical iron-caused visible light spectrum.

The conclusion is obvious: this gem is a natural sapphire, and the condition of the included crystal means the sapphire has not undergone high-temperature heat treatment. A complete identification was possible in this case with just two instruments. An added bonus in this instance was that no refractometer was needed, which saves the contact surface of the hemicylinder for when it is actually needed.

Other examples of the spectroscope’s utility are in the determination of dyed green jadeite from natural Cr-colored jadeite, and in the detection of irradiated yellow diamonds, where the instrument is of crucial value. There is really no other way for a gemologist to get this important information except through spectroscopic examination.

The three major types of visible spectra found in corundum, as viewed through a direct-vision prism vs. diffraction-grating spectroscope. Illustration from Ruby & Sapphire by Richard W. Hughes

Pattern recognition

Whether you have a small pocket-sized diffraction-grating spectroscope without a built-in scale, or a table-top prism spectroscope with a built-in numerical scale, using a spectroscope is simply about pattern recognition. No matter which instrument you use, the positions of the absorption lines and bands will always be the same for any particular gem species that shows an absorption spectrum. As for example, iron lines will always be in the same areas of the blue region in an iron spectrum of a sapphire. The absorption pattern shown by a ruby will always be the same and in the same position in the visible light spectrum.

Source: www.lotusgemology.com
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