Gemology Fast Track / Unit 17 of 24 · Major species · 60 min

Jade, turquoise, lapis, pearl, amber, coral

Goal: Handle ornamental and organic gems: spot RI, SG, treatments and imitations.

Jadeite

Spot 1.66 · SG 3.34 · 437 nm line. Type A/B/C grading is everything. Imperial green top value.

Nephrite

Spot 1.61 · SG 2.95. Felted fibres: toughest gem. Mutton-fat white valued in China.

Turquoise

Spot 1.61 · SG 2.6–2.9. Most stabilised. Dyed howlite/magnesite imitations everywhere.

Lapis lazuli

Spot 1.50 · SG 2.75. Lazurite + pyrite + calcite. Sodalite is darker, lighter SG, no pyrite.

Pearl

SG 2.66–2.78. Tooth test gritty. Natural vs cultured = lab X-ray.

Amber

SG 1.08: floats in saturated salt water. Copal tacky with acetone.

Jadeite treatment types

Type A

Natural, wax only.

Type B

Acid bleached + polymer. Etched 'spider-web' surface, dull ring when tapped, FTIR polymer peaks.

Type C

Dyed: colour in veins between grains; no Cr lines in spectrum.

B+C

Both. Very common in markets.

Pearl grading (7 factors)

LustreSurfaceShapeColour & overtoneSizeNacre quality / thicknessMatching (strands)
AkoyaBead-nucleated, 6–8 mm typical, sharp lustre.
South SeaWhite/golden, 9–20 mm, satiny.
TahitianNatural dark colours; red LWUV fluorescence.
FreshwaterMostly solid nacre, many shapes and dyes.

Quick tests for organics (non-destructive first)

Brine floatAmber floats, plastic usually sinks.
Hot point (hidden spot)Amber: pine; copal: resin; plastic: acrid; black coral/tortoiseshell: burnt hair; jet: coal. Destructive — last resort.
MagnificationCoral: parallel wavy striations; ivory: Schreger lines; pearl: nacre platelets 'contour' pattern.

Legal: ivory, tortoiseshell and some corals are CITES-restricted. Don't buy or ship without paperwork.

Gems in this unit (22)
JadeiteNephriteTurquoiseLapis lazuliNatural pearlAkoya pearlSouth Sea pearlTahitian pearlFreshwater cultured pearlKeshi pearlMabe pearlConch pearlMother-of-pearlImitation pearlAmberDominican blue amberCopalPrecious coralBlack coralImitation turquoisePlasticSynthetic turquoise

Full gem records open in the members' Gem database.

Practical
Test any jade-like object: spot RI (if refractometer) or SG by hydrostatic weighing (works well on jade carvings). 3.34 → jadeite, 2.95 → nephrite, 2.57 → serpentine.
Unit quiz

0/7 answered

1. Jadeite SG is about…
2. Type B jadeite is…
3. A blue 'turquoise' whose colour comes off on an acetone swab:
4. Natural vs cultured pearl needs…
5. Amber vs plastic in saturated brine:
6. Lapis vs sodalite:
7. Toughest gem material:
Unit 16Unit 18