How to Identify a Gemstone: Six Measurable Tests

Nobody identifies a gemstone by looking at it and guessing. Every confident identification rests on measurements — optical and physical properties that a stone cannot fake, whatever its colour or cut. The JDMIS gem reference organises 229 gem varieties into 11 colour-and-clarity families, from "Red, Pink and Purple — Transparent" to "Grey and Black", and then separates look-alikes with six measurements. Here is how each one works, with real values for stones you actually meet in Singapore workshops.

1. Start with colour and clarity

Colour comes first because it is the fastest filter. A transparent red stone is already down to a shortlist — ruby, spinel, garnet, tourmaline — before any instrument is switched on. Clarity matters just as much: the same red family splits into transparent and translucent to opaque branches, and the two branches contain different stones. Colour plus clarity puts you in the right neighbourhood; the measurements that follow tell you which house it is.

2. Single or double refraction?

Light splits into two rays inside some crystals and not others. A polariscope shows this in seconds: diamond, spinel and cubic zirconia are singly refractive, while ruby, sapphire, topaz, tourmaline and quartz are doubly refractive. That single observation rules out half a shortlist. A red stone that stays dark through a full rotation under the polariscope is not a ruby — it is behaving like a spinel or a garnet.

3. Measure the refractive index

The refractometer is the workhorse of gem testing. You place a polished facet flat on the glass prism, look through the eyepiece, and read where the shadow edge falls on a scale — the refractive index (RI).

• Diamond: 2.417
• Cubic zirconia: 2.15
• Ruby and sapphire: 1.762–1.77
• Spinel: 1.718
• Topaz: 1.619–1.627
• Amethyst and citrine: 1.544–1.553

A standard gem refractometer reads up to about 1.79–1.81. Diamond and cubic zirconia sit over the limit — the shadow edge runs off the top of the scale, which is itself a diagnostic result, and one reason CZ became such a convincing diamond simulant at first glance.

4. Read the birefringence

In a doubly refractive stone, the two rays travel at different speeds. The gap between them — the birefringence — is the difference between the highest and lowest RI the stone shows. Amethyst reads 1.544 to 1.553, a birefringence of 0.009; tourmaline spans 1.624 to 1.644, up to about 0.018 and beyond. In strongly birefringent stones you can even see facet-edge doubling with a loupe. The number is stable for each species, so it separates stones whose RI ranges overlap.

5. Check pleochroism with a dichroscope

Doubly refractive stones can show different colours along different crystal directions. A dichroscope — two small windows that show the two polarised rays side by side — makes this obvious and repeatable. Alexandrite shows strong green, orange and pink pleochroism; tourmaline is typically moderate to strong; amethyst is weak to moderate in purple tones. Singly refractive stones like spinel never pleochroise, so a dichroscope also double-checks your step-2 conclusion. Related to this is optic character: quartz and corundum are uniaxial, topaz and chrysoberyl biaxial — a finer split used when the numbers alone do not decide.

6. Fluorescence under ultraviolet light

Switch on a long-wave and short-wave UV lamp and some stones glow. Burmese ruby fluoresces strong red in long-wave UV; light Ceylon sapphires show moderate to strong blue; amethyst is usually inert, at most a weak blue in short-wave; alexandrite ranges from inert to a moderate red. Fluorescence never identifies a stone on its own — the same species can glow differently depending on its chemistry — but as a confirming signal it is fast and non-destructive.

And the tie-breaker: specific gravity

Density settles the cases the optical tests leave open. The hydrostatic method needs just a scale and a cup of water: weigh the stone in air, then suspended in water, and the two weighings give you specific gravity (SG). Diamond sits at 3.51–3.53 while cubic zirconia — visually its most convincing impersonator — is far heavier at 5.6–6. Topaz (3.49–3.57) and sapphire (3.95–4.1) can look alike in a tray; on a scale they are different stones. Quartz at 2.64–2.69 is one of the lightest gems you will handle.

Practise the whole sequence yourself

Reading numbers is one thing; running the sequence on a real stone until it becomes instinct is another. If you want to build that instinct:

• The GT100 Gem and Jewellery Trade Secrets course at JDMIS teaches the complete foundation — the stones, the tools and the trade context. You can preview free lessons from GT100 on the JDMIS learning platform before enrolling.
• The Gem Identification Methods course (GT200) takes you deeper into hands-on testing of unknown stones.
• JDMIS also runs a free online Gem Reference with an identification wizard: enter what you observe — colour, refraction, RI, UV reaction, specific gravity — and it narrows 229 varieties down to the stones that match your measurements. It is the same colour-first, measurement-driven logic this article walks through.

Six measurements, one sequence, no guessing. That is how a professional turns "a red stone" into a name.

Want the equipment view first? See How to Identify Gemstones at Home: 8 Tools That Work for what each instrument looks like and what it tells you.