Gemology Fast Track / Unit 14 of 24 · Major species · 60 min

Tourmaline, topaz, zircon

Goal: Identify three often-confused species and their treatments.

Tourmaline

1.624–1.644RI
0.018–0.020U−
3.06SG

Strong dichroism, darker along c-axis. Doubling visible. Rubellite Type III.

Topaz

1.609–1.637RI
0.008–0.010B+
3.53SG

Perfect basal cleavage, slippery feel, very clean. Blue = irradiated.

Zircon

1.925–1.984RI (OTL)
0.059U+
4.70SG

Huge doubling, adamantine, chipped edges. 653.5 nm line. Not cubic zirconia.

Tourmaline trade names

Rubellite — red, stays red in all lightsIndicolite — blueVerdelite — greenParaíba — Cu-bearing neon blue/greenChrome — Cr/V greenWatermelon — pink core, green rimCanary — Mn yellowAchroite — colourlessSchorl — blackDravite — brown

Orientation matters (cutting rough)

c table ∥ c → lighter table ⟂ c → darker (looks down c)

Dark green/blue tourmaline: put the table parallel to the c-axis to avoid the dark 'closed' c-axis colour. Pale tourmaline: table across the axis to intensify colour.

Same logic for kunzite (best colour down c) and sapphire (o-ray purer blue down c).

Topaz treatments

Irradiation + heatColourless → Sky/Swiss/London blue. Must pass residual radioactivity checks before sale.
HeatOrange imperial → pink.
Coating'Mystic' rainbow, Cr-diffusion pinks: check wear at facet edges.

Zircon

High

Crystalline. RI 1.925–1.984, SG ~4.7.

Low (metamict)

Radiation-damaged: RI 1.78–1.85, SG ~4.0, nearly SR, green.

Blue zircon

Heated from brown Cambodian rough. Some revert slightly; avoid long UV.

Gems in this unit (26)
TopazoliteRubellitePink tourmalineIndicoliteGreen tourmalineChrome tourmalineParaíba tourmalineWatermelon tourmalineSchorlDraviteLiddicoatiteCat's-eye tourmalineAchroiteCanary tourmalineImperial topazBlue topazPink topazColourless topazMystic topazBrown topazBlue zirconColourless zirconRed zirconYellow zirconLow zirconCubic zirconia

Full gem records open in the members' Gem database.

Practical
In the Database, compare Topaz vs Tourmaline vs Danburite vs Andalusite (all RI 1.61–1.65). Write the one property that separates each pair.
Unit quiz

0/7 answered

1. Topaz bire and optic character:
2. The c-axis in tourmaline is usually…
3. Paraíba tourmaline's colour comes from…
4. Blue topaz colour is almost always from…
5. Zircon's diagnostic spectrum line:
6. Cubic zirconia and zircon are…
7. Topaz durability concern:
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