Topaz

Topaz species including imperial, blue, sherry, and colourless varieties with properties, treatments, and identification.

By Fabian Moor Last updated
species/topaz imperial-topaz blue-topaz sherry-topaz

Introduction

Topaz (Al₂SiO₄(F,OH)₂) is an aluminium fluorosilicate crystallising in the orthorhombic
system, with hardness 8 Mohs, RI 1.619–1.627 (biaxial positive, birefringence 0.008–0.010),
SG 3.50–3.57, and perfect basal cleavage in one direction, a durability liability
despite high hardness. [1] Blue topaz dominates the commercial
market but is nearly always irradiated colourless topaz; London Blue requires neutron
irradiation followed by a hold period for radioactive decay before retail. [2]
Natural blue is very rare and pale. Imperial topaz (vivid orange to pink-orange, coloured
by chromium and colour centres, from Ouro Preto, Minas Gerais, Brazil) commands premium
prices far above treated blue; fine saturated reddish-orange stones with strong
three-axis pleochroism are genuinely rare above 5 carats. Sherry topaz (yellowish-brown
to orange-brown) may fade on prolonged light exposure, unlike the stable irradiation-
produced blues. Topaz SG of 3.53 virtually overlaps diamond (3.52), making hydrostatic
separation between the two unreliable. [1]

Mineralogy

Crystal System and Structure

  • Crystal system: Orthorhombic
  • Chemical formula: Al₂SiO₄(F,OH)₂
  • Habit: Prismatic crystals, often striated
  • Cleavage: Perfect basal (one direction)
  • Fracture: Conchoidal

Physical Properties

Property Value
Hardness 8 Mohs
Specific gravity 3.50–3.57
Refractive index 1.619–1.627
Birefringence 0.008–0.010
Optic character Biaxial positive
Pleochroism Weak to distinct (varies by colour)
Lustre Vitreous

Cleavage Warning

Colour Varieties

Imperial Topaz

The most valuable topaz variety:

  • Colour: Orange to pink-orange to reddish-orange
  • Source: Primarily Ouro Preto, Brazil [1]
  • Cause: Chromium in some; colour centres
  • Rarity: True imperial colour is rare
  • Value: Commands significant premiums

The term "imperial" should be reserved for the finest orange to
pinkish-orange colours, not ordinary yellow or golden topaz.

Sherry Topaz

Yellowish-brown to orange-brown:

  • Colour: Named for sherry wine colour
  • Sources: Brazil, Sri Lanka, Pakistan
  • Note: May fade with light exposure [1]
  • Value: Lower than imperial; higher than colourless

Blue Topaz

The most common market topaz today:

  • Natural: Rare and typically pale
  • Treated: Virtually all commercial blue topaz [2]
  • Treatment: Irradiation + heat
  • Trade names: Sky Blue (light), Swiss Blue (medium), London Blue (dark)

Pink Topaz

  • Natural: Extremely rare
  • Treated: Heat-treated from brownish or yellowish material
  • Source: Brazil, Pakistan (natural pink rare)
  • Value: High for natural; moderate for treated

Colourless Topaz

  • Abundance: Common; often treated to blue
  • Historic: Used as diamond simulant
  • Value: Lowest of topaz varieties
  • Uses: Starting material for treatment

Blue Topaz Treatments

Understanding treated blue topaz:

Treatment Process

Virtually all blue topaz on the market is treated:

  1. Start with colourless topaz (abundant)
  2. Irradiate (various methods produce different blues)
  3. Heat to stabilize and develop colour
  4. Hold for radiation decay (if neutron-irradiated)

Blue Topaz Types

Trade Name Colour Treatment
Sky Blue Light, clear blue Electron or gamma irradiation
Swiss Blue Medium saturated blue Combination irradiation
London Blue Deep greyish-blue Neutron irradiation + heat [2]

Safety Note

Neutron-irradiated topaz (London Blue) must be held for radiation
decay before sale. [2] Reputable suppliers ensure this, but the delay
period can be months.

Major Sources

Origin Characteristics
Brazil (Ouro Preto) Imperial topaz; finest orange to pink-orange
Brazil (general) Blue treatment material; various colours
Pakistan Pink, golden, some fine imperial
Sri Lanka Various colours; colourless
Russia (Urals) Historic; blue and pink
USA (Utah, Texas) Collector specimens; some gem quality

Characteristic Inclusions

Topaz typically has good clarity; when inclusions occur:

  • Two-phase inclusions: Liquid with gas bubble
  • Growth tubes: Parallel to c-axis
  • Cleavage planes: May show iridescence
  • Needle inclusions: Fine parallel needles
  • Crystal inclusions: Various minerals

Historical Notes

Identification Summary

Key features for topaz identification:

  • RI: 1.619–1.627 (useful range)
  • SG: 3.53 (overlaps diamond!) [1]
  • Birefringence: 0.008–0.010
  • Optic sign: Biaxial positive [1]
  • Cleavage: Perfect basal (visible under magnification)
  • Spectrum: Often no diagnostic features

Distinguishing Similar Gems

Gem Key Distinction from Topaz
Citrine Lower RI (1.544–1.553), lower SG (2.65)
Aquamarine Lower RI (~1.58), lower SG (2.72)
Yellow sapphire Higher RI (1.76–1.77), higher SG (4.00)
Danburite Lower SG (3.00), different crystal system
Heliodor Lower RI (~1.58), lower SG (~2.72)

References

  1. ↑ 1. Read, P. (2008). Gemmology (3rd ed.). Butterworth-Heinemann. ISBN: 978-0-7506-6449-3. DOI: 10.4324/9780080507224.
  2. ↑ 2. Ashbaugh, C. (1988). Gemstone Irradiation and Radioactivity. Gems & Gemology, 24(4), 196–213. DOI: 10.5741/gems.24.4.196.

Related Topics

Physical PropertiesOptical PropertiesGemstone Treatments