A coloured stone is coloured because it absorbs some wavelengths of light and passes the rest. The spectroscope shows exactly which wavelengths are absorbed, as dark lines and bands across a rainbow, and because each colouring element absorbs in its own pattern, the spectrum is a signature. It is one of the quickest ways to tell what is making a stone the colour it is — and, sometimes, that the colour was not made the way the seller says.
How it works
The hand spectroscope is a small tube containing either a diffraction grating or a train of prisms. Light is transmitted through or reflected off the stone and into the instrument, which spreads it into a spectrum from red at one end to violet at the other. Where the stone absorbs, the spectrum is crossed by dark lines (sharp, from specific electronic transitions) or bands (broad regions of absorption). The examiner reads their positions against a wavelength scale or against memory of reference spectra.
The test takes seconds, damages nothing, and works on rough, beads and cabochons as well as faceted stones — anything light can pass through.
Characteristic spectra
Chromium. The colouring element of ruby, emerald, red spinel, alexandrite and some green garnet. Its spectrum has a distinctive pattern: fine lines in the deep red (a doublet in ruby that appears bright in fluorescing stones), a broad absorption in the yellow-green, and lines in the blue. Seeing the chromium spectrum in a red stone confirms ruby or red spinel; seeing it in a green stone confirms emerald or chrome-bearing material.
Iron. Blue and green sapphire, aquamarine, peridot, almandine garnet. Sapphire shows bands in the blue (around 450 nm); almandine garnet shows three strong bands in the yellow-green that identify it at once; peridot shows a three-band iron spectrum in the blue.
Cobalt. Blue glass and synthetic blue spinel show three broad, strong bands in the orange, yellow and green that no natural blue sapphire shows. A blue stone with a cobalt spectrum is glass or synthetic spinel — an instant separation.
Rare earths (didymium). Sharp, fine groups of lines in the yellow and green, seen in yellow apatite, some synthetic materials and certain glasses.
Zircon. A ladder of many fine, sharp lines through the whole spectrum from uranium, characteristic and unmistakable in most zircon.
Dyed material. Dyed green jadeite and dyed green quartz show a broad band in the red that natural chromium-coloured jadeite does not, separating dyed from natural in seconds. Dyed material generally shows broad, diffuse absorption that does not correspond to any natural colouring element.
What the spectroscope decides
It tells the examiner what is producing the colour. That answers several questions: whether a red stone's colour comes from chromium (ruby, spinel) or iron (garnet) or nothing recognisable (glass); whether a blue stone is coloured by iron and titanium (sapphire) or cobalt (imitation); whether green colour is chromium (emerald) or a dye. It is a strong test for imitations and for dye, and a supporting test for species.
It does not separate natural from synthetic ruby — both have chromium — and it does not detect heating. Those remain microscope questions. How a gemmological laboratory tests a gemstone.
Limits
Reading a hand spectroscope takes practice; the lines can be faint, and pale stones show little. Dark stones may need strong light. Some species show no useful spectrum at all. Modern laboratories complement the hand instrument with digital spectrometers (UV-Vis-NIR, FTIR, Raman) that record the spectrum precisely and extend into wavelengths the eye cannot see, which is where advanced treatment detection lives.
Frequently asked questions
Can the spectroscope identify a stone by itself?
For some — almandine garnet, zircon, cobalt glass — the spectrum is diagnostic on its own. For most it confirms the colouring element and is combined with RI and SG.
Does it work on beads and cabochons?
Yes. It needs only light passing through the stone, not a flat facet, which makes it valuable for the articles the refractometer cannot read.
Can it detect dye?
Often, particularly in green jadeite and quartz, where dye produces absorption that natural colour does not.
Does it separate natural and synthetic?
Not generally. A synthetic ruby has the same chromium spectrum as a natural one. Some synthetics show additional or missing features that help, but the microscope is the primary tool.
Is a digital spectrometer better?
More precise and more sensitive, and it records a permanent trace. The hand instrument is faster and needs no setup; laboratories use both.