Topaz
Topaz species including imperial, blue, sherry, and colourless varieties with properties, treatments, and identification.
Introduction
Topaz (Al₂SiO₄(F,OH)₂) is an aluminium fluorosilicate prized for its excellent
hardness, brilliance, and variety of colours. While blue topaz dominates the
commercial market, the rare imperial topaz from Brazil commands premium prices.
Topaz has been known since antiquity, though historically the name was applied
to various yellow gems including peridot and citrine.
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
- 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
- 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
- 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
Blue Topaz Treatments
Understanding treated blue topaz:
Treatment Process
Virtually all blue topaz on the market is treated:
- Start with colourless topaz (abundant)
- Irradiate (various methods produce different blues)
- Heat to stabilize and develop colour
- 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 |
Safety Note
Neutron-irradiated topaz (London Blue) must be held for radiation
decay before sale. 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
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) |