Gemology Fast Track / Unit 7 of 24 · Instruments · 60 min

Polariscope, dichroscope and filters

Goal: Separate SR / DR / aggregate, find optic figures and use pleochroism and the Chelsea filter.

Polariscope reactions (turn the stone 360° between crossed filters)

Stays darkSingly refractive (or on optic axis). Spinel, garnet, glass, diamond.
Light ↔ dark 4× per turnDoubly refractive. Ruby, quartz, tourmaline, topaz.
Stays lightAggregate: jade, chalcedony; or very included DR stone.
Patchy, snake-like bandsAnomalous double refraction (strain): synthetic spinel ('tabby'), garnet, glass, diamond.

Interference figures (conoscope sphere or glass rod on top)

UniaxialBlack cross with colour rings — corundum, beryl, tourmaline.
BiaxialCurved brushes (isogyres) around two eyes — topaz, peridot, tanzanite.
Bull's-eyeColoured centre, no cross in the middle — quartz (optical activity).
Airy spiralTwinned quartz (amethyst Brazil-law twinning) — supports natural.

Dichroscope (calcite / London type)

Use

Hold close to the eye, stone lit from behind with white light. Two windows side by side show two ray colours. Rotate.

Same colour in both windows always

SR, or looking down an optic axis. Try other directions.

Two/three colours

DR. Ruby: purplish red / orangy red. Iolite: violet-blue / yellow / grey. Tanzanite: blue / violet (/ brown unheated).

Chelsea colour filter

How

Strong incandescent light on the stone, filter close to eye, 30 cm away. Passes only deep red + yellow-green.

Turns red/pink

Most Cr: emerald (not all), ruby, red spinel, alexandrite, chrome tourmaline, chrome diopside, tsavorite. Cobalt blue: synthetic blue spinel, cobalt glass.

Stays green/grey

Most green tourmaline, peridot, green glass, green beryl, Fe-rich emeralds (some Zambian/Brazilian). Aquamarine green vs blue spinel/glass red.

CF reactions are clues, never proof. Dyed green chalcedony and some dyed jade also go red.

Practical
Make a simple polariscope: two polarising sunglass lenses (or a phone screen as the bottom polariser + one sunglass lens on top). Test a glass bead (dark/ADR), a quartz crystal (blinks) and a chalcedony bead (stays light).
Unit quiz

0/7 answered

1. A gem stays light through full rotation in the polariscope. Most likely…
2. Snake-like patchy extinction indicates…
3. A bull's-eye figure identifies…
4. Same colour in both dichroscope windows in all directions:
5. Under a Chelsea filter, aquamarine looks… while blue synthetic spinel looks…
6. A Chelsea filter result alone…
7. Biaxial interference figure shows…
Unit 6Unit 8