Two of the simplest instruments in a gem laboratory are also two of the most efficient. Neither gives a number. Each asks a yes-or-no question about how the stone handles polarised light, and each answer rules out a large part of the field.
The polariscope
Two polarising filters, one above the other, set at right angles so that no light passes between them. A stone is placed between the filters and rotated through 360°.
Singly refractive stones — diamond, garnet, spinel, glass, plastic, opal, and every amorphous or cubic material — stay dark throughout the rotation. Light enters, passes through unchanged, and is blocked by the second filter.
Doubly refractive stones — corundum, quartz, beryl, tourmaline, topaz, zircon, chrysoberyl, and every other crystal that is not cubic — split light into two rays with different polarisation. As the stone rotates, it goes light and dark four times in a full turn. This "blinking" is unmistakable.
Strained glass and some other materials show an irregular, patchy or snake-like pattern that shifts as the stone turns — anomalous double refraction. It never resolves into the clean four-fold blinking of a true crystal, and once seen it is recognised. Many glass imitations are caught by this alone.
Aggregates — chalcedony, jadeite, nephrite, lapis, turquoise — are made of countless tiny crystals in random orientation and stay uniformly light throughout: the aggregate reaction.
The polariscope is why a red stone that reads as singly refractive is immediately not ruby (which blinks), and why a "sapphire" that shows anomalous strain is glass. Add a small glass sphere or a conoscope lens above a doubly refractive stone and the polariscope shows an interference figure — a pattern of rings and crosses that tells whether the stone is uniaxial or biaxial. A quartz sphere gives a "bull's-eye" figure that identifies quartz outright. Quartz vs glass: how to tell real crystal from imitation.
The dichroscope
A small tube with a calcite rhomb or two polarising filters side by side, showing two images of the same spot on the stone in two polarisation directions. If the stone transmits different colours in different directions — pleochroism — the two windows show different colours.
Only doubly refractive coloured stones can be pleochroic, and each species has its own pair (or trio) of colours:
| Stone | Pleochroic colours |
|---|---|
| Ruby | purplish red / orangey red |
| Blue sapphire | violet-blue / greenish blue |
| Emerald | bluish green / yellowish green |
| Tourmaline (green) | dark green / yellow-green, often strong |
| Iolite | violet-blue / pale blue / pale yellow (three colours, very strong) |
| Tanzanite | blue / purple / brownish (three) |
| Andalusite | green / reddish brown |
| Amethyst | purple / reddish purple (weak) |
| Kunzite | pink / violet / colourless |
Singly refractive stones — garnet, spinel, glass — show the same colour in both windows, always. Aggregates show none.
The dichroscope separates ruby from red garnet and red glass, sapphire from blue spinel and blue glass, emerald from green garnet and green glass, and iolite from anything. It also flags dye: a dyed stone's colour is the same from every direction, because dye in cracks has no crystallographic orientation.
Using the two together
A red stone: polariscope shows blinking (doubly refractive — not garnet, spinel or glass); dichroscope shows purplish red and orangey red (chromium-coloured corundum). Twenty seconds, and the field is narrowed to ruby, natural or synthetic — which the refractometer confirms and the microscope then sorts.
A blue stone: polariscope stays dark (singly refractive); dichroscope shows one colour. Not sapphire. Spinel, glass or something else, which RI and SG decide.
Limits
Neither instrument works well on opaque stones or on very dark stones that pass little light. Pleochroism can be weak and hard to see in pale stones. And the tests separate categories, not individuals: a doubly refractive, pleochroic red stone is corundum-like, but natural versus synthetic is beyond either instrument. How a gemmological laboratory tests a gemstone shows where they fit.
Frequently asked questions
Can I make a polariscope at home?
Two polarising filters (from photographic supplies, or even two lenses from polarised sunglasses) crossed at right angles with a light beneath will show the blinking. It is a useful hobby tool.
Does pleochroism affect appearance?
In strongly pleochroic stones, yes — a tourmaline or iolite can look different colours from different angles, and cutters orient the stone to show the best colour face-up.
Can glass be doubly refractive?
Glass is amorphous and is singly refractive, but internal strain from cooling can produce anomalous double refraction — a patchy effect, not the clean blinking of a crystal. Recognising the difference takes a little experience.
Do synthetics show the same reactions as naturals?
Yes. Synthetic ruby blinks and shows the same pleochroism as natural ruby. These tests identify the material, not its origin.
Why do reports mention "optic character"?
Uniaxial or biaxial, positive or negative — derived from the polariscope interference figure or the refractometer — is a further property that narrows the species and is recorded on detailed reports.