Gemologists identify stones by measuring how they handle light, how heavy they are for their size and what they contain inside. The everyday kit includes a loupe, microscope, refractometer, polariscope, dichroscope, spectroscope, UV lamp and scales. Advanced labs add spectrometers and chemical analyzers that can separate natural from synthetic stones, detect treatments and suggest a geographic origin.
Loupe
A 10x loupe is a folding magnifier and the most used tool in the trade. It is the standard magnification for diamond clarity grading, so a flaw that cannot be seen at 10x does not count against a diamond's clarity grade. Good loupes use a triplet lens, three glued elements that correct for color fringes and distortion at the edges. With a loupe, a gemologist can check surface polish, facet junctions, inclusions, chips and signs of treatment such as filled cracks or glassy residues.
Gemological microscope
A gemological microscope is a binocular stereo microscope, usually zooming from about 10x to 60x or more, with special lighting. Darkfield lighting makes inclusions glow against a black background. Brightfield lighting shows color zoning and curved growth lines. Fiber-optic lights can pick out surface features. Many microscopes have a stone holder, and some have an immersion cell that holds the gem in liquid to cut surface reflections.
The microscope is where most identification and treatment detection happens. It can reveal curved striae in flame-fusion synthetic corundum, gas bubbles in glass, flux inclusions in synthetic emerald, flash effects in fracture-filled diamond and the glassy residues left by heat treatment or lead-glass filling.
Refractometer
A refractometer measures refractive index (RI), the amount light slows and bends inside the gem. The gem's flat facet sits on a glass hemisphere with a drop of contact liquid between them. Looking through the eyepiece, the user reads a shadow edge on a scale. A doubly refractive stone shows two readings as it is rotated, and the difference is its birefringence.
Standard refractometers read up to about 1.79 to 1.81, limited by the RI of the contact liquid. Diamond, zircon, demantoid and some garnets are above this limit and show no reading. Curved surfaces need a spot reading, where the gemologist looks at a tiny drop of liquid on the cabochon.
Polariscope
A polariscope has two polarizing filters, one below the stone and one above. With the filters crossed, the field is dark. Turning the stone between them shows whether it is singly refractive (stays dark), doubly refractive (blinks light and dark four times per turn) or an aggregate (stays light). Strain in glass or synthetic spinel shows as odd patterns. With a conoscope, a small glass sphere held above the stone, the gemologist can see interference figures that show whether a stone is uniaxial or biaxial.
Dichroscope
A dichroscope shows pleochroism, different colors seen in different directions. The calcite type uses a calcite rhomb to split light into two images side by side. Looking at a gem through it, a doubly refractive colored stone can show two different colors at once. Ruby shows purplish red and orangy red. Tanzanite and iolite show strong differences. Singly refractive gems such as garnet and spinel show the same color in both windows.
Spectroscope
A hand spectroscope spreads white light into a rainbow, and a gem in the light path removes certain colors. The dark bands, called absorption lines, are linked to particular elements. Chromium gives ruby, red spinel and emerald fine lines in the red. Iron gives almandine garnet and blue sapphire bands in the green and blue. Zircon often shows many fine lines from traces of uranium. Didymium glass has a well-known line pattern that helps identify glass imitations.
Ultraviolet lamp
A UV lamp shines longwave (365 nm) and shortwave (254 nm) ultraviolet light on a gem to check for fluorescence. Ruby from low-iron marble deposits glows red. Many diamonds glow blue under longwave UV, and a group of diamonds glowing the same color can be a quick way to check a parcel for mixed material. Synthetic stones and some treatments can show different reactions from their natural counterparts. Shortwave UV harms eyes and skin, so lamps are used in a viewing cabinet or with eye protection.
Chelsea color filter
The Chelsea filter is a small dark filter that passes only deep red and yellow-green light. Developed in London in the 1930s, it was first used to help separate emerald from its common imitations. Many emeralds containing chromium appear red or pink through it, while green glass and many simulants appear green. Results vary by source and are only a quick screen, so the filter is used alongside other tests.
Scales and specific gravity
An electronic carat scale weighs gems to 0.01 or 0.001 carat. Fitted with a hydrostatic attachment, the same scale can measure specific gravity: the gem is weighed in air and then in water, and the results give its density relative to water. Heavy liquids, which float some stones and sink others, are an older way to compare density, but many are toxic and they are used less often today.
Thermal and electrical testers
Thermal conductivity testers, often called diamond testers, measure how fast a heated metal tip loses heat to the stone. Diamond conducts heat very well and reads differently from nearly every simulant. Moissanite also conducts heat well and can pass a thermal test, so multi-testers add an electrical conductivity check, since most moissanite conducts electricity and most diamonds do not. Natural blue diamonds containing boron also conduct electricity, which can cause confusion. The page on moissanite covers this. Neither tester can separate natural diamond from lab-grown diamond.
Diamond screening devices
Because lab-grown diamonds have the same properties as natural ones, separating them needs special tools. Screening devices use UV transparency, fluorescence and phosphorescence patterns or spectroscopy to sort diamonds into "pass" and "refer for further testing." Larger labs use imaging systems that show the growth patterns of natural, HPHT and CVD diamonds under shortwave UV. Lab-grown diamonds explains why this matters for buyers.
Advanced laboratory tools
Major gem labs rely on instruments that most jewelers do not own:
| Instrument | What it measures | Typical uses |
|---|---|---|
| FTIR spectrometer | Absorption of infrared light | Diamond type (Ia, IIa and so on), resin and oil fillings in emerald, polymer impregnation in jade |
| UV-Vis-NIR spectrometer | Absorption from ultraviolet to near infrared | Color causes, heat treatment clues in corundum, diamond color treatment |
| Raman spectrometer | How a laser's light is scattered | Fast identification of minerals and inclusions, without damaging the gem |
| Photoluminescence (PL) | Light emitted under laser excitation, often at low temperature | Natural versus treated or lab-grown diamond |
| EDXRF | X-ray fluorescence from elements | Non-destructive chemistry, such as checking for lead glass or copper in Paraiba-type tourmaline |
| LA-ICP-MS | Trace elements by removing a tiny amount of material with a laser | Origin studies, beryllium diffusion detection in sapphire, copper and gallium levels |
| X-radiography and micro-CT | Internal structure | Natural versus cultured pearls |
LA-ICP-MS removes a speck of material too small to see without magnification. Labs use it on corundum, emerald and other gems where trace elements carry clues about treatment and origin.
Matching tools to questions
No single instrument identifies every gem. A gemologist builds a case: RI and birefringence narrow the list, specific gravity and optic character confirm it, and the microscope and spectroscope reveal whether the stone is natural, synthetic or treated. Origin and some treatments need lab spectroscopy and chemistry. The guide on how to identify gemstones walks through this process, and diamond grading labs covers who does the advanced work.
Frequently asked questions
What is the most important gemology tool?
The 10x loupe is used most, but the refractometer and microscope do most of the identification work for colored stones.
Can a diamond tester tell if a diamond is lab-grown?
No. Thermal and electrical testers separate diamond from simulants such as cubic zirconia and moissanite, but natural and lab-grown diamond test the same. Separating them needs screening devices or lab analysis.
Why can't a refractometer read diamond?
Diamond's RI of about 2.42 is higher than the contact liquid, which limits standard refractometers to readings up to about 1.79 to 1.81.
What does the Chelsea filter show?
Many chromium-colored emeralds look red or pink through it, while most green glass and imitations look green. It gives a quick hint, not a final answer.