Gemology Fast Track / Unit 6 of 24 · Instruments · 60 min

The refractometer

Goal: Take flat-facet and spot readings, find birefringence and optic character.

1.762 / 1.770 ruby: two shadow edges

How it works

Light hits the stone–glass contact. Above the gem's critical angle it's totally reflected; the scale shows a light area and a shadow edge at the stone's RI.

Standard range 1.30–1.81 (limited by the contact liquid, RI ~1.79–1.81). Above that: OTL (over the limit) — diamond, zircon, demantoid, CZ, sphene.

Flat-facet method

1Clean table and prism. Tiny drop of contact liquid on the prism (~1 mm).
2Slide the stone table-down onto the drop near the centre.
3Look through the eyepiece with a polarising filter. Read the shadow edge(s) to 0.001.
4Rotate the stone in 45° steps through 180°, reading high and low each time.
5Highest − lowest = birefringence. Behaviour of the edges = optic character.

Interpreting the shadow edges

One edge, never moves

Singly refractive (or optic axis view). Spinel 1.718, garnets, glass.

Two edges, one fixed

Uniaxial. Fixed edge = ω (ordinary). If moving edge is above fixed → U+; below → U−.

Two edges, both move

Biaxial. If the middle value (β) is closer to the low → B+; closer to the high → B−.

Blurry single reading

Cabochon or aggregate: use the spot method.

Spot (distant vision) method for cabochons

Put a tiny dot of liquid on the prism, touch the cab's dome to it. Back away from the eyepiece ~30 cm; you'll see an oval spot. Move your head until the spot is half light / half dark. Read that line: e.g. jadeite 1.66, nephrite 1.61, chalcedony 1.53–1.54, turquoise 1.61.

Safety

Standard RI liquid contains diiodomethane + sulphur; it's toxic and stains. Use a tiny drop, wipe immediately, wash hands, store dark (it yellows).

Damage risk

Never slide a hard stone across the glass prism: diamond, corundum and moissanite can scratch it. Place, don't drag.

RI ladder of key gems

Practical
If you have a refractometer: measure a quartz (1.544/1.553, bire 0.009, U+). Rotate 180° and plot both edges at each 45° step. If you don't, use the members' ID Helper and practise: enter 1.762/1.770, bire 0.008, U− and see what comes up.
Unit quiz

0/7 answered

1. A stone shows one edge at 1.718 that never moves. It is probably…
2. Two edges, one fixed at 1.544, the other moves up to 1.553. Optic character:
3. Readings 1.762 and 1.770 give birefringence…
4. 'OTL' means…
5. The spot method is used for…
6. Both shadow edges move as you rotate. The gem is…
7. Spot reading 1.66 on a green cabochon points to…
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