Loupewise / Guide / How to Use a Spectroscope
Spectroscope: a raked cylindrical barrel on a wedge stand, with an eyepiece at the high end and a round stage at the foot

How to Use a Spectroscope for Gemstone Identification

Last updated: August 2026

The most decisive cheap instrument in gemology, and the hardest to get a result from. What it decides, the two lighting methods, and the stones to practise on first.

A spectroscope shows you which wavelengths a stone eats. Colouring elements absorb precise slices of the spectrum, so a stone's absorption pattern — dark bands and lines across the rainbow — points at its chemistry in a way no other bench observation can. Anderson called it the third leg of the tripod alongside the refractometer and the microscope, and its territory is exactly where those two give up: it works on rough as easily as on cut stones, and on material far past the refractometer's limit.

The patterns are the strongest single observations in Loupewise's engine. Of 132 species, seeing the 653 nm line pair leaves 3 candidates; the 415 nm line leaves 5; the iron bands of peridot leave 10; the almandine triple leaves 11; manganese bands leave 14; the chromium doublet leaves 24. Nothing else in the kit cuts that deep in one look.

And it is the hardest instrument here to get a result from. The problem is light: a diffraction grating spreads the spectrum evenly but passes far less light than a prism, and the brightness of the spectrum is, in Anderson's words, of prime importance. Most beginner failures with a spectroscope are lighting failures. The elimination numbers above assume a pattern correctly seen — this page is about making that happen.

Grating or Prism

Diffraction grating

Even spread of colours across the whole range, with a generally larger view of the red — but a series of spectra form on either side of the beam, so far less light reaches your eye in the one you watch. Usually a fixed slit: nothing to adjust, nothing to clean.

This is the type most beginners own, and the dim image is a property of the design, not a fault in yours.

Prism

A brighter spectrum — Anderson recommended a small prism scope for fast spotting for exactly that reason — but the spread is uneven: blue stretched out, red compressed. Often an adjustable slit, which brings its own discipline (below).

Some models carry a built-in wavelength scale; on grating models the scale, where present, reads in nanometres.

Before Any Gemstone: Calibrate Your Eye

Point the spectroscope at light sources whose spectra are already structured — a fluorescent tube, a computer monitor. You will see clear bright and dark structure immediately, which teaches you what a band looks like, how to hold the instrument steady, and what your particular scope's spectrum looks like when something real is there. Do this before you ever try a stone; a beginner who has never seen a certain band cannot judge a faint one.

Then adopt one habit from Anderson: always view the spectrum the same way round — he preferred red at the left — so band positions become spatial memory rather than a fresh puzzle each time.

The Two Lighting Methods

Both methods share one rule: the light should be as strong as you can arrange, and none of it should reach your eye except through the stone. Glare beside the stone swamps the faint spectrum you are trying to read.

1

Reflected light — the everyday method

Stone table-down on black velvet. Light enters the pavilion at about 45°, and the spectroscope sits at 45° on the other side, slit a few inches from the stone — the light takes its longest path through the stone and picks up the most colour. The black, non-reflective background matters: it stops the surface under the stone contributing its own reflected light to your reading. This method shows weak bands in pale stones at their best, and shows the bright fluorescence lines of ruby and red spinel well.

2

Transmitted light — for deep colour and the blue end

Light passes straight through the stone into the slit — the simplest form is stone and penlight in one hand, illuminating the gem from behind. Anderson's refinement is to fill the field with an even, out-of-focus glare of transmitted light (he used a microscope's condenser under the stone and rested the spectroscope where the eyepiece goes). Reflected light is not good for deep-coloured stones or for bands in the blue and violet; transmitted light is how you reach those.

3

Match the method to the stone

This choice is itself a skill: a pale stone read in transmitted light can show nothing while its bands are plainly there in reflected light, and a deep stone read by reflection can be a black smear that opens up in transmission. Seeing no bands with one method is a reason to try the other, not a result.

Slit Discipline (Adjustable Models)

Start with the slit fairly wide — around a quarter of a millimetre — and narrow it as you refine the view. And learn the one distinction that saves wrong readings: absorption bands cross the spectrum vertically; streaks that run along it are dirt on the slit, and usually clear if you open the slit slightly (or clean it gently with a sharpened matchstick). Fixed-slit grating scopes are spared all of this.

The Practice Stones the Sources Themselves Name

Anderson's list of easy first spectra, unchanged since Church first observed absorption bands in 1866: Ceylon zircon (brown or greenish), almandine garnet, and synthetic ruby — each with a well-defined pattern that a beginner can find. All three are in Loupewise's database and drill pool, so you can practise the recognition on cards before ever pointing the instrument at a stone, then confirm your first real observations on species where the answer is known.

One trap on ruby specifically: ruby and red spinel emit bright fluorescence lines in the red as well as absorbing elsewhere. A bright line is not a dark band — Anderson's copper-sulphate-flask trick existed precisely to see those emission lines clearly against darkness. If a feature in the red looks brighter than the spectrum around it, you are looking at fluorescence, and it is diagnostic in its own right.

The five patterns, at the positions the rows record One shared scale, 400–700 nm. Every mark is a value from the species' own database row. Zircon 8 recorded features 516537562589615621653.5659 Almandine Garnet 4 recorded features 504520527573 Ruby 5 recorded features 476620659692.8694.2 Peridot 3 recorded features 453473493 Spinel 5 recorded features 533580619684685 400 450 500 550 600 650 700 nm absorption line — width follows the row's strength broad band bright fluorescence line DO Practise on these five before anything unknown — the guide's own sources name them as the easy first spectra. SEE Zircon's staircase runs eight recorded features from 516 to 659 nm around its strong 653.5 line; almandine's three strong bands sit at 504, 520 and 573 nm; ruby pairs bright emission at 692.8/694.2 nm with broad absorption; peridot's iron trio steps 453, 473, 493 nm; spinel lines the red at 685/684. MEANS Match the whole pattern, never one line. These positions and strengths are the same values the app narrows with — the rows they come from cite their sources. Feature positions and strengths are each species' own gems.json values (claim class physical_properties, cited to the row's sources); nothing here is drawn from memory. The band colouring is Dan Bruton's published piecewise approximation of visible wavelengths (1996), gamma 0.8 — a depiction convention, not colourimetry. Mark widths follow the row's strength word by a stated convention; a broad band's drawn extent is a convention of ±14 nm, because the rows record a centre and the word "broad", never a width. Ruby's 692.8/694.2 pair is drawn bright because its row records fluorescence lines — see the guide's own note above: a bright line is not a dark band. Drawn span 400-700 nm.

Where It Goes Wrong

Too little light — the central failure, and the grating's dimness makes it worse. Stronger source, tighter geometry, longer path through the stone.

Glare beside the stone — light reaching the eye around the specimen washes out faint bands. Anderson boxed his lamp so no light escaped except through the stone; your version is careful geometry and a shaded lamp.

The background reads as the stone — anything reflective under the specimen adds its own light to your spectrum. Black velvet is the sourced fix.

Streaks read as bands — dirt on the slit runs along the spectrum; real bands run across it.

Wrong method for the stone — see step 3 above; a null result with one lighting method is not a null result.

Emission read as absorption — ruby's bright red lines are fluorescence, not bands.

No lines is also a reading These three are not failures of technique — their rows record clean spectra in so many words. Danburite 0 placeable features “a clean featureless spectrum” — its own row Fluorite 0 placeable features “typically featureless in the visible range” — its own row Topaz 0 placeable features “spectrum generally featureless” — its own row 400 450 500 550 600 650 700 nm DO Calibrate on a known lined stone first, so you can trust an absence when you see one. MEANS A clean band can be the stone's true reading: 76 of 132 species' rows record no feature a hand spectroscope can place. Rule out the failure list above — light, glare, background — before crediting the stone with a clean spectrum. The three rows quoted are verbatim from gems.json — the generator asserts each phrase appears in the species' own spectroscope_absorption entry, and that each species sits in the recorded-clean set (at least one recorded token, zero placeable features). The 76-of-132 count is computed from the corpus at generation, not typed. Band colouring is the same Bruton (1996) depiction convention as the figure above; drawn span 400-700 nm.

Loupewise takes the pattern you saw — the 653 nm pair, the 415 nm line, iron bands, the chromium doublet and more — and shows which of 132 species remain consistent with it.

Open Loupewise

Studying for the FGA or GIA practical? Loupewise drills spectrum recognition alongside instrument readings, and runs timed mock exams built around the real exam formats.

Practice exams →

Where This Comes From

Frequently Asked Questions

Why can't I see anything through my spectroscope?

Almost always light. A grating scope passes little of it, and a dim, diffuse source gives you a spectrum too faint to read. Move to a stronger lamp, tighten the geometry so light reaches your eye only through the stone, and try both lighting methods — then check your technique on a fluorescent tube, which shows unmistakable structure if the instrument and your eye are working.

Which spectroscope should a beginner buy — grating or prism?

Either genuinely works; they trade against each other. The grating gives an even spectrum with a better view of the red and needs no slit maintenance, at the cost of a dimmer image. The prism is brighter — Anderson's preference for quick spotting — but compresses the red, stretches the blue, and its adjustable slit needs discipline. What matters more than the choice is the lighting technique on this page.

What stone should I practise on first?

Synthetic ruby, then almandine garnet, then a brown or greenish zircon — the sources' own list. Synthetic ruby is cheap, its spectrum is strong, and it carries the instructive trap: bright fluorescence lines in the red that teach you the difference between emission and absorption on day one.

Do I need a wavelength scale?

No. The classic patterns are recognised by their shape and their neighbourhood — a pair of lines deep in the red, a triple of bands across the middle, a lone line at the violet end — and that is how Loupewise's drill cards teach them. A scale helps you talk about what you saw; it is not what lets you see it.