Spectroscope BasicsSpectroscope Basics

Gemology Tools Mini Course

Spectroscope Basics

IGS may receive customer referral fees from the companies listed in this page.

So you've just bought your first spectroscope. Now what do you do? How do you use it for analyzing and identifying gemstones? Although based on sophisticated science, the spectroscope itself is a simple device that needs only a few adjustments to use. These general spectroscope instructions will help hobbyists and novice gemologists get familiar with operating and calibrating this instrument.

Purchase Gemology Tools Mini Course

Do all the different tools in gemology confuse you? Whether you’re just setting up your at-home gem lab or an old pro who needs a refresher, you’ll love this course. Learn everything you need to know about standard gemology tools, from how they work to how to use them.
synthetic ruby
Ruby gemstones are ideal for calibrating a spectroscope's scale. Our basic spectroscope instructions will show you how to do this. A synthetic ruby, like this brilliant-cut stone, is an affordable option for novice gemologists. Photo by James St. John. Licensed under CC By 2.0.

What is a Spectroscope?

The spectroscope is an important tool for identifying gemstones. Since gemstone species have characteristic structural and chemical properties, gems selectively absorb specific wavelengths of visible light. Gemologists use spectroscopes to view the absorption spectra of unknown gemstones and compare those patterns to those of known gemstone species.

Anyone interested in advanced spectroscope instructions and gem identification techniques should consider enrolling in the IGS Professional Gemologist Certification Course. The curriculum includes an in-depth article on this instrument.

Examine Your Spectroscope

Spectrum - spectroscope instructionsTake your new spectroscope, point it towards a window or bright light, and look through the eyepiece. You should see a rectangular-shaped rainbow spectrum, like the one pictured to the right. (Make sure you're looking through the correct end).

All spectroscope models have ways of adjusting the focus and the amount of light entering the instrument. If you have an advanced spectroscope, you should also see a scale. If it's not immediately visible, try moving your head slightly to either side. Advanced models also have ways to adjust the focus and position of a scale for identifying light wavelengths. Find the scale adjustment and turn it. You will see the scale move from side to side.

Find a Gemstone's Absorption Lines

Basic prism spectroscope
Basic prism spectroscope

Place your spectroscope and a gem of known identity in the spectroscope's stand. Use a gemstone with a strong, distinctive absorption spectrum, like a ruby, apatite, zircon, or peridot. If your model doesn't have a stand, you can try some alternative techniques.

Next, adjust the amount of light entering the spectroscope. Slowly turn the light opening. Watch the spectrum through the eyepiece as you do this. As you adjust the light opening, the absorption lines — dark lines on the spectrum — will come into view. Try different settings until the absorption lines are most clearly visible.

If you can't find the absorption lines, make sure the light is passing through the gem and into the end of your spectroscope. Turn off all other room lights, so they're not affecting your reading.

If you're still having trouble finding the absorption lines, you may not be looking straight down the tube. Set the light opening to the middle position and move your head slightly from side to side until you see the lines.

Focus Your Spectroscope

Once you find the absorption lines, focus the spectroscope. Depending on your model, you can do this by sliding a tube in and out or by turning a ring. You'll notice that the absorption lines don't become clear all at once. When one end of the spectrum is in focus, the other end will be slightly fuzzy. You may need to readjust the focus to see all the significant absorption lines.

Calibrate Your Spectroscope's Scale

If you have an advanced spectroscope with a scale, focus your instrument until the scale is clear and easy to read.

Ruby Absorption SpectrumRuby displays strong double absorption lines at 475 and 476.5 and at 692.8 and 694.2 nanometers (nm). Choose a strong line, such as 475 nm. Adjust the scale position until the 475 marker lines up with the strong absorption line you are viewing. The scale markers at 476.5, 692.8, and 694.2 nm should then line up with the other strong absorption lines. Adjust the scale until the markers are on the known strong lines of the ruby absorption spectrum.

You've now properly calibrated your scale. However, you may need to make subtle adjustments to the scale when you view other stones. You can always use a gemstone with distinctive absorption lines to recalibrate your scale if it's moved inadvertently.

International Gem Society

Never Stop Learning

When you join the IGS community, you get trusted diamond & gemstone information when you need it.

Become a Member

Get Gemology Insights

Get started with the International Gem Society’s free guide to gemstone identification. Join our weekly newsletter & get a free copy of the Gem ID Checklist!