Gemological Instruments: A Guide to the Tools That Identify Gems

What are gemological instruments?

Gemological instruments (often spelt gemmological in Australia) are the tools a gemologist uses to identify a gemstone, separate natural stones from lab-created ones and look for treatments. Most of them measure a physical or optical property: how strongly the stone bends light, how dense it is, which colours of light it absorbs and what is trapped inside it. Each property narrows the list of possibilities until only one gem fits.

Identification matters because many gems look alike. A blue stone could be sapphire, spinel, tanzanite, indicolite tourmaline, glass or a lab-created sapphire, and colour alone will not tell them apart. Our guides to individual coloured gemstones describe each species in detail; this page explains the instruments behind the identification notes in those guides. If a term is new to you, the GemTrove jewellery glossary defines it.

The instruments fall into two groups. The standard gem-testing kit (loupe, refractometer, polariscope, dichroscope, spectroscope, UV lamp, microscope and a balance) fits on a bench and identifies most gems. Advanced laboratory instruments, such as infrared, Raman and photoluminescence spectrometers and laser chemical analysis, are used by grading labs for treatments, geographic origin and lab-grown diamond detection.

Which gemological instruments does a gemologist use most?

The loupe is the first and most-used tool. A good one is a triplet: three lenses bonded together so the image has no colour fringes (chromatic aberration) and no distortion at the edges. 10x is the standard because it is the magnification used for diamond clarity grading: if a trained grader cannot see a feature at 10x, it does not count against the clarity grade. Fine-tipped locking tweezers and a daylight-balanced lamp complete the basic kit.

None of these tools works alone. A gemologist builds a profile, RI plus optic character plus spectrum plus inclusions, and only calls an identification when several independent readings agree.

How does a refractometer identify a gemstone?

Every transparent gem slows light down and bends it as it enters. The refractive index (RI) is the number for how much: water is about 1.33, quartz about 1.54, sapphire about 1.76 and diamond 2.42. Because RI depends on a gem's chemistry and crystal structure, it is one of the most useful single readings in gemology.

A refractometer has a small hemisphere of high-index glass set into its top. The gemologist puts a drop of contact liquid (RI about 1.79–1.81) on the glass, lays a flat polished facet on the drop and looks through the eyepiece. A shadow edge appears across a scale, usually read under yellow light near 589 nm, and the edge sits at the gem's RI. For cabochons and curved surfaces, the spot method gives an approximate reading to two decimal places.

Doubly refractive gems split light into two rays, so they show two shadow edges that shift as the stone is turned. The gap between the highest and lowest readings is the birefringence. Corundum has a small birefringence of about 0.008; tourmaline, with a gap of around 0.018–0.020, shows two clearly separate edges; and peridot's unusually large birefringence of about 0.036 is one of its signature readings. How the edges move on rotation also shows whether a gem is uniaxial (one optic axis) or biaxial (two), which separates species with similar RIs.

The refractometer's limit is the contact liquid. Anything above about 1.81 reads as OTL (over the limit): diamond, moissanite, cubic zirconia and high-type zircon, whose RI of about 1.93–1.98 sits off the scale. OTL is itself a clue, and the other instruments take over from there. At the low end, citrine and every other variety of quartz read a tight 1.544–1.553 with 0.009 birefringence, which separates citrine quickly from yellow glass.

How do gemstone readings compare on the instruments?

Gems that look identical often separate cleanly here. A blue spinel and a blue sapphire can be exactly the same shade, but spinel gives one reading near 1.718 and sapphire two readings around 1.76–1.77. Tanzanite and blue sapphire are also confused across the counter; tanzanite's RI is lower and its SG is much lower, 3.35 against 4.00.

Specific gravity (SG) compares a gem's weight with the weight of the same volume of water. On a hydrostatic balance, SG = weight in air ÷ (weight in air − weight in water). A 4.00 sapphire feels noticeably heavier in the hand than a 2.66 quartz of the same size, and an SG reading settles cases where two gems have overlapping RIs.

Hardness on the Mohs scale is a scratch ranking only. For rings, toughness (resistance to chipping and breaking) matters as much. Corundum and spinel are hard and tough, which is why they suit everyday rings. Zircon is hard enough but brittle on its facet edges, and tanzanite (Mohs 6–7, with one direction of perfect cleavage) is better in protected settings or pieces worn less often.

What do a polariscope and dichroscope show?

A polariscope has two polarising filters, one below the stone and one above, turned so that no light passes through them (crossed). The gemologist rotates the stone between them through a full 360° and watches what happens:

• Singly refractive (SR): stays dark all the way round. Diamond, garnet, glass and spinel, a singly refractive gem often mistaken for sapphire behave this way.
• Doubly refractive (DR): blinks light and dark four times per full turn. Sapphire, tourmaline, peridot and quartz all do this.
• Aggregate: stays light throughout, because the material is made of many tiny crystals. Jadeite and chalcedony are examples.
• Anomalous double refraction (ADR): patchy, snake-like dark bands caused by internal strain. Common in lab-created spinel and some garnets, and a reminder that one reading is never enough.

Adding a conoscope, a small glass sphere held above the stone, turns the light into an interference figure: a dark cross with coloured rings for uniaxial gems, one or two curved bars for biaxial gems. That can separate a uniaxial gem from a biaxial look-alike even when the refractometer cannot be used.

Many coloured doubly refractive gems show different colours in different directions, called pleochroism (dichroism for two colours, trichroism for three). A calcite dichroscope places two colours side by side in a small window, which makes even subtle differences obvious. Tanzanite's three body colours of blue, violet and bronze are a textbook case. Strong pleochroism is also why blue indicolite tourmaline looks darker down its length; cutters usually orient the table so the lighter direction faces up. Sapphire shows blue against greenish blue, while singly refractive spinel shows only one colour.

The Chelsea colour filter lets through only deep red and yellow-green light. Chromium-coloured emerald often glows red through it, and cobalt-coloured lab-created blue spinel and cobalt glass turn red, while natural blue sapphire stays dark. It is a quick screening tool, not proof.

How does a spectroscope help identify gemstones?

A handheld spectroscope spreads white light into a rainbow from violet (about 400 nm) to red (about 700 nm). When light passes through a gem, dark lines or bands appear wherever an element absorbs it. These patterns work like fingerprints for the colouring elements:

• Iron in blue sapphire: a band at 450 nm, often with weaker lines at 460 and 470 nm. Australian sapphires from Anakie in Central Queensland and the New England fields of New South Wales formed in iron-rich, basalt-related settings, and many show a strong 450 nm band.
• Chromium in ruby and pink sapphire: a fine red doublet near 694 nm, broad absorption across the yellow-green and fine lines in the blue. In pink sapphire, where chromium gives the colour, the same lines appear more faintly.
• Yellow sapphire: iron-rich yellow sapphire from Australia and Thailand usually shows the 450 nm iron lines, while low-iron Sri Lankan yellows often show little. That hints at the geological source but does not prove origin.
• Zircon: many natural zircons show a sharp line near 653.5 nm, caused by trace uranium.
• Peridot: three evenly spaced iron bands in the blue, near 453, 473 and 493 nm.

Australia has a long connection with zircon. Jack Hills in Western Australia has yielded zircon grains around 4.4 billion years old, among the oldest known pieces of Earth's crust, and gem zircon turns up in the same gravels as sapphire in the eastern Australian fields.

Ultraviolet lamps work alongside the spectroscope. Long-wave UV (365 nm) and short-wave UV (254 nm) make some gems fluoresce, meaning they glow visibly. Chromium-rich ruby glows red, many natural diamonds glow blue under long-wave, and some lab-grown diamonds show unusual short-wave reactions or a lingering afterglow called phosphorescence. Iron suppresses fluorescence, so iron-rich Australian sapphire is usually inert. Never look directly at a short-wave lamp; it can damage your eyes.

What can a gemological microscope reveal about inclusions?

The microscope is where most identifications are confirmed. A gemological microscope is a stereo zoom instrument, usually about 10x to 60x, with darkfield illumination: light enters from the sides against a black background, so inclusions glow brightly against the dark. Fibre-optic lights, polarising filters and immersion in liquid help reveal colour zoning and growth lines.

Inclusions, the crystals, fluids and growth features trapped inside a gem, record how it formed. Natural and lab-created stones usually look quite different under magnification:

• Natural sapphire and ruby: fine needles of rutile called silk, angular colour zoning and small mineral crystals. Aligned silk is what creates the six-rayed star in star sapphires and star rubies.
• Heat-treated corundum: partly dissolved silk, crystals surrounded by disc-shaped stress fractures, and altered fingerprints.
• Flame-fusion lab-created corundum and spinel: curved growth lines, like the grooves on a record, and round gas bubbles.
• Flux-grown lab-created stones: wispy flux fingerprints and sometimes metallic platelets from the crucible.
• Tourmaline: thin liquid-filled tubes and fractures. Rubellite tourmaline, which is rarely free of inclusions, is judged on whether they are eye-visible rather than absent.
• Peridot and zircon: disc-shaped 'lily pad' inclusions in peridot, and strong doubling of the back facets in zircon that shows even under a loupe.

Magnification also shows colour zoning clearly. In watermelon tourmaline, with its pink centre and green rim, the zoning is the whole appeal of the stone. In a sapphire, sharp straight bands of colour support a natural origin, while curved bands point to a flame-fusion lab-created stone.

How do labs detect treatments and lab-created gems?

Some questions cannot be answered with the bench kit. Is a yellow sapphire's colour natural or beryllium-diffused? Is a tourmaline coloured by copper? Is a diamond natural or lab-grown? For these, gem laboratories use instruments that are expensive and need specialists to interpret.

Trace chemistry is how labs handle Paraíba tourmaline and its copper colour. The name is now used for copper-bearing tourmaline whatever the source, so a report may call a stone Paraíba and give the geographic origin, such as Brazil, Nigeria or Mozambique, as a separate opinion. Origin is always an expert opinion based on comparing inclusions and chemistry with reference samples, never a certainty.

Some treatments leave no reliable trace even in a lab. Most morganite is gently heated to remove a yellow tint, and this is generally undetectable, so it is assumed. Nearly all tanzanite is heated as well. Treatments like these are stable and widely accepted; what matters is that they are disclosed.

Can a diamond tester tell lab-grown from natural diamond?

No. A handheld thermal conductivity tester measures how quickly heat moves through the stone. Diamond conducts heat extremely well, so the tester separates it from glass, cubic zirconia and most other simulants. Lab-grown diamond is real diamond with the same thermal properties, so it tests as diamond, exactly as it should.

Moissanite (silicon carbide, which in jewellery is lab-created) also conducts heat well and can pass a thermal-only tester. Combined testers add an electrical conductivity check, because moissanite conducts electricity and most diamond does not. The exception is blue boron-bearing Type IIb diamond, natural or lab-grown, which also conducts and can read as moissanite. Under a loupe, moissanite gives itself away with doubled facet edges, because it is doubly refractive while diamond is singly refractive.

Separating laboratory-grown from natural diamond needs screening devices and spectroscopy. Most natural diamonds are Type Ia, with nitrogen in clusters that shows up in infrared and UV absorption; most lab-grown diamonds are Type IIa or IIb, with little or no nitrogen. Screening instruments pass Type Ia stones as natural and refer the rest for laboratory tests such as photoluminescence and growth-pattern imaging, because a small share of natural diamonds are Type IIa too. That is why a diamond should come with a report from a lab such as GIA or IGI stating whether it is natural or lab-grown.

How does a gemologist identify a gemstone? Step by step

1. Look first. Note colour, lustre, transparency, any phenomenon such as a star or colour change, and how the stone is cut and worn.
2. Use the 10x loupe. Check for doubled back facets, gas bubbles, curved lines, chipped facet edges and surface fillers.
3. Take a refractometer reading. Record the RI and birefringence, or note OTL for high-index stones.
4. Check the polariscope. Decide whether the gem is singly refractive, doubly refractive or an aggregate.
5. Use the dichroscope and filter. Look for pleochroic colours and any Chelsea filter reaction.
6. Read the spectrum and fluorescence. Look for iron, chromium or other lines, then test under long- and short-wave UV.
7. Weigh it for SG if the stone is loose and the readings still overlap.
8. Confirm under the microscope. Inclusions settle natural versus lab-created and reveal many treatments.
9. Send it to a lab when the question is origin, a hard-to-detect treatment or whether a diamond is natural or lab-grown.

A mounted stone limits what can be tested: the refractometer needs a facet clear of metal, and SG cannot be measured at all. When you look at a stone or a report, it is reasonable to ask which tests were done and what the identification rests on.