Gemology Fast Track / Unit 5 of 24 · Instruments · 60 min

Loupe, light and inclusions

Goal: Hold a loupe properly, light a stone three ways and name what you see.

The loupe

10× triplet (corrected for colour and distortion), ~20 mm lens. Industry standard for clarity grading.

Holding

Loupe close to the eye (2–3 cm). Brace hands together, stone in tweezers at ~2.5 cm from lens. Move the stone, not your head.

Tweezers

Locking, fine point, grooved. Hold the stone at the girdle, table up or table down.

Three lighting setups

Darkfield

Light from the side against a dark background. Inclusions glow bright. Best for clarity.

Brightfield

Light from below through the stone. Shows colour zoning, curved striae, dark crystals.

Surface reflection

Light bounces off facets: polish, scratches, coatings, filled fractures with lower lustre.

Inclusion vocabulary (and what it tells you)

Needles / silkRutile in corundum & garnet. Intact silk → usually unheated.
FingerprintHealed fracture of tiny fluid droplets. Natural in many species; flux fingerprints in synthetics.
FeatherOpen fracture. Durability risk; check if it reaches the surface.
CrystalIncluded mineral. Garnet in diamond, calcite in Colombian emerald, chromite in peridot.
Two-phaseLiquid + gas bubble in a cavity.
Three-phaseLiquid + gas + crystal (salt cube): Colombian emerald.
Colour zoningStraight/angular = natural growth; curved = flame-fusion synthetic.
Gas bubblesRound bubbles = glass or flame-fusion synthetic.
HorsetailCurved chrysotile fibres: demantoid (Russian/Italian).
Lily padDiscoid stress fracture around a crystal: peridot; also heated corundum.
Treacle / heat waveSwirly look: hessonite garnet.
Flash effectBlue/orange flashes in fractures: glass or resin filling.

Clarity types for coloured stones (GIA)

Type I

Usually eye-clean: aquamarine, topaz, tanzanite, chrysoberyl, morganite.

Type II

Usually some inclusions: ruby, sapphire, garnet, peridot, spinel, tourmaline (non-red), quartz.

Type III

Almost always included: emerald, red/pink tourmaline (rubellite).

A microscope (10–60× zoom, darkfield base) shows the same things far better. Once you're doing clarity grading or treatment detection every week, it's the first big upgrade after the loupe.

Practical
Examine three stones under the loupe in darkfield (lamp from the side, black card behind) and brightfield (white card under). Sketch each stone's inclusions as top-view plots and name each using the vocabulary above.
Unit quiz

0/7 answered

1. Round gas bubbles in a red stone suggest…
2. Curved growth lines (striae) indicate…
3. Three-phase inclusions are classic in…
4. Best lighting to see inclusions glow against a dark ground:
5. Emerald is a clarity Type…
6. Horsetail inclusions identify…
7. Blue/orange flashes in fractures indicate…
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