Identifying a gemstone is a process of elimination. Every mineral has a fixed set of physical and optical properties — how strongly it bends light, how dense it is, how it behaves between crossed polarising filters, which colours it absorbs — and no two gem species share all of them. A laboratory measures enough of these properties to leave only one candidate, and then examines the stone under magnification to decide whether it is natural, synthetic or treated.
None of this requires cutting, scratching or heating the stone. The whole sequence is non-destructive, which is why a stone can be tested, reported and worn without any change.
Step 1 — intake and documentation
Before any test, the stone is weighed on a carat balance (to 0.001 ct on a laboratory balance, or 0.01 ct for routine work), measured with a digital gauge, described in plain language — colour, shape, cutting style, transparency — and photographed. These details identify the specific stone the report is about. A report without weight and measurements cannot be matched to a stone and is of little use.
Step 2 — refractive index
The refractometer is the workhorse of gem identification. A flat facet of the stone is placed on a glass hemicylinder with a drop of contact liquid, and the instrument shows the angle at which light is totally reflected. That angle converts to the refractive index (RI): a number, typically between 1.4 and 1.8, that is characteristic of the species.
Some examples the examiner keeps in mind: quartz 1.544–1.553, beryl (emerald, aquamarine) about 1.577–1.583, corundum (ruby, sapphire) 1.762–1.770, garnet from about 1.73 to 1.89 depending on type, spinel about 1.718. Glass varies widely with its composition, which is itself a clue.
The refractometer also shows birefringence — whether the stone gives one reading or two. A stone with two readings is doubly refractive (most crystalline gems); one reading means singly refractive (diamond, spinel, garnet, glass) or a very small birefringence. Where two readings appear, their difference and behaviour as the stone is rotated give the optic character. A full account is in Refractive index: the most important number in gem identification.
Stones with an RI above the refractometer's limit (about 1.81) — diamond, zircon, some garnets — read "over the limit", which is itself diagnostic.
Step 3 — specific gravity
Specific gravity (SG) is the stone's density relative to water. The laboratory weighs the stone in air, then suspended in water, and calculates the ratio. Corundum is about 4.00, quartz 2.65, diamond 3.52, emerald about 2.72, glass anywhere from 2.3 to over 4 depending on lead content.
SG is especially useful for stones that cannot be tested on the refractometer — beads, cabochons with no flat facet, carvings and rough. It is also a quick check against imitations: a "ruby" with an SG of 2.5 is not corundum, whatever it looks like. See Specific gravity: weighing a stone against water.
Step 4 — polariscope and dichroscope
Between two crossed polarising filters a singly refractive stone stays dark as it is turned, while a doubly refractive stone blinks light and dark every 90 degrees. Glass and other amorphous materials can show a patchy "strain" pattern that is instantly recognisable. The polariscope is a ten-second test that separates whole families of possibilities.
The dichroscope shows whether a coloured stone displays two (or three) different colours when viewed along different directions — pleochroism. Ruby shows purplish-red and orangey-red; iolite shows violet, blue and pale yellow; glass and garnet show only one colour. Pleochroism is a strong clue to species and cannot be imitated by dye.
Step 5 — the microscope
Everything so far identifies the species. The microscope answers the question buyers actually care about: is this stone natural, synthetic or treated?
At 10× to 60× the examiner looks at inclusions — the crystals, needles, fluids and growth features trapped inside the stone as it formed. Natural stones show the signatures of geological growth: angular mineral crystals, healed fractures like fingerprints, silk (fine rutile needles) in corundum, three-phase inclusions in Colombian emerald. Synthetics show the signatures of the factory: curved growth lines and gas bubbles in flame-fusion material, chevron growth and flux residues in flux-grown stones, distinctive seed plates in hydrothermal material.
Treatments also leave traces. Heat alters or destroys silk and can leave a discoid fracture around a crystal that expanded. Glass filling in ruby shows gas bubbles in the fractures and a blue flash effect at the filling boundary. Oil and resin in emerald show a flash of colour at the fissure. Dye collects in cracks and grain boundaries. Inclusions: the fingerprints inside a gemstone goes deeper.
Step 6 — spectroscope and ultraviolet
The hand spectroscope spreads the light passing through the stone into a rainbow and shows dark absorption lines where the stone swallows particular wavelengths. Chromium-coloured stones (ruby, emerald, red spinel) show a characteristic pattern in the red; cobalt glass shows three broad bands that no natural blue stone matches; zircon shows a fine ladder of lines. The pattern is a fingerprint of the colouring agent and often the species.
Under long-wave and short-wave ultraviolet lamps, many stones fluoresce in ways that help separate natural from synthetic, or treated from untreated — synthetic ruby often glows more strongly than natural, glass-filled fractures can show a different reaction from the host, and some diamond simulants are easily identified this way. The reaction is always read alongside the other tests, never alone.
Step 7 — advanced testing (when needed)
For some questions — whether a sapphire has been heated at low temperature, whether a diamond is HPHT-grown, the geographic origin of a fine ruby — the standard bench is not enough, and larger laboratories use spectrometers (FTIR, UV-Vis, Raman) and trace-element analysis. A good routine laboratory knows the limits of its instruments and says so on the report: "no indications of heating observed" is an honest statement; "guaranteed unheated" from a bench without spectroscopy is not.
Step 8 — the report
The findings are written up: species and variety, weight, dimensions, colour, shape, transparency, the measured RI and SG, the tests carried out, any treatment detected or its absence, and a conclusion. A serious report states what was measured rather than what was assumed, and it draws a line between what the tests support and what they do not. What a laboratory report cannot tell you covers that boundary.
Frequently asked questions
Can a gemstone be identified by eye?
Experienced examiners can make good guesses, and most of the time the guess is right. But glass imitates every colour, synthetics are chemically identical to the natural stone, and treatments are invisible to the naked eye. Identification without instruments is opinion; with instruments it is measurement.
Does testing damage the stone?
No. The refractometer needs a drop of contact liquid that wipes off; SG uses water; the microscope, spectroscope and UV lamp only shine light. The stone leaves the laboratory exactly as it arrived.
How long does identification take?
A routine identification takes minutes to half an hour on the bench. Treatment and origin questions that need advanced instruments can take longer, and some cannot be answered with certainty at all.
Can a lab tell if a stone is synthetic?
Almost always, yes, by its inclusions and growth features — a synthetic stone has the same chemistry but grew in a very different environment, and that environment leaves marks. Some very clean synthetics require advanced instruments.
What is the difference between identification and grading?
Identification says what the stone is. Grading assesses quality — colour, clarity, cut — against a scale. A laboratory report is primarily identification; any grading is an opinion and should be labelled as such.