Zircon
Zircon species including blue, colourless, and fancy colours with metamict types, properties, treatments, and identification.
Introduction
Zircon (ZrSiO₄) is a zirconium silicate with exceptional brilliance and
dispersion. Often confused with cubic zirconia (CZ), natural zircon is a
completely different material—one of the oldest minerals on Earth, with
specimens dating back 4.4 billion years.
The December birthstone in its blue form, zircon offers diamond-like
sparkle at a fraction of the price.
Mineralogy
Crystal System and Structure
- Crystal system: Tetragonal
- Chemical formula: ZrSiO₄
- Habit: Tetragonal prisms with pyramidal terminations
- Cleavage: Imperfect
- Fracture: Conchoidal
Physical Properties
| Property | High Zircon | Low Zircon |
|---|---|---|
| Hardness | 7.5 Mohs | 6.0 Mohs |
| Specific gravity | 4.6-4.7 | 3.9-4.1 |
| Refractive index | 1.925-1.984 | 1.78-1.85 |
| Birefringence | 0.059 | Lower to nil |
| Dispersion | 0.039 (high) | Lower |
Metamict Zircon
Zircon contains trace radioactive elements (uranium, thorium) that cause
internal radiation damage over geological time.
The Metamict Process
- Radioactive decay damages crystal structure
- Progressive breakdown from crystalline to amorphous
- Properties change as structure deteriorates
- Process takes millions to billions of years
Zircon Types
| Type | Crystal State | Properties |
|---|---|---|
| High zircon | Fully crystalline | Full properties; best for gems |
| Intermediate | Partially damaged | Intermediate properties |
| Low zircon (metamict) | Largely amorphous | Reduced RI, SG, birefringence |
Restoration
Heat treatment can restore metamict zircon:
- Heating to ~1000°C can recrystallize the structure
- Restores high RI, SG, birefringence
- Also changes colour in many cases
- Process is permanent and stable
Colour Varieties
Blue Zircon
The most popular zircon colour:
- Origin: Heat treatment of brown zircon
- Colour: Light to vivid blue
- Treatment: Universal (virtually all blue zircon is heated)
- Stability: Permanent
- Sources: Cambodia, Sri Lanka
Colourless Zircon
Excellent diamond simulant:
- High dispersion: 0.039 (approaches diamond's 0.044)
- High RI: Creates strong brilliance
- Historic use: Diamond substitute before CZ
- Distinction from diamond: Lower hardness, high birefringence
Other Colours
| Colour | Notes |
|---|---|
| Golden/yellow | Natural or heat-treated |
| Brown | Common natural colour; starting material |
| Green | Often low/metamict zircon |
| Red | Rare; highly valued |
| Orange | Attractive variety |
High Birefringence
Blue Zircon Treatment
Understanding blue zircon treatment:
Treatment Process
- Brown Cambodian/Sri Lankan zircon selected
- Heated in reducing atmosphere (oxygen-free)
- Temperatures around 900-1000°C
- Colour change to blue occurs
- Process also restores metamict material
Stability
- Colour is permanent
- Does not fade with light or heat
- Treatment universally accepted
- Disclosure standard but not always practiced
- Assume all blue zircon is heated
Characteristic Inclusions
Zircon Halos
Sources
| Origin | Characteristics |
|---|---|
| Cambodia (Ratanakiri) | Premier blue zircon source |
| Sri Lanka | All colours; historic source |
| Myanmar | Red, golden, colourless |
| Australia | Ancient zircons; scientific importance |
| Tanzania | Various colours |
| Vietnam | Blue, golden |
Identification Summary
Key features for zircon identification:
- RI: 1.925-1.984 (high; often over limit)
- SG: 4.6-4.7 (high)
- Birefringence: 0.059 (very high; visible doubling)
- Dispersion: 0.039 (high fire)
- Spectrum: Characteristic uranium lines throughout
- Doubling: Diagnostic with loupe
Distinguishing Similar Gems
| Gem | Key Distinction from Zircon |
|---|---|
| Diamond | Diamond is SR (no doubling); higher hardness |
| Cubic Zirconia | CZ is SR; higher SG (5.6-6.0) |
| Sapphire | Sapphire has lower RI (1.76-1.77) |
| Topaz | Topaz has lower RI (1.62), lower SG (3.53) |