Feldspar Group
Feldspar group gems including moonstone, labradorite, sunstone, amazonite, and orthoclase with properties, phenomena, and identification.
Introduction
Feldspars are the most abundant minerals in Earth's crust, and several members
produce important gem materials. The group is notable for spectacular optical
phenomena including adularescence (moonstone), labradorescence, and aventurescence
(sunstone).
Feldspars divide into two main series: alkali feldspars (potassium/sodium)
and plagioclase feldspars (sodium/calcium).
Feldspar Classification
Alkali Feldspars
Potassium-sodium feldspars:
- Orthoclase (KAlSi₃O₈): Includes moonstone
- Sanidine: High-temperature form
- Microcline: Includes amazonite
Plagioclase Feldspars
Sodium-calcium series:
- Albite (NaAlSi₃O₈): Sodium end-member
- Labradorite: Intermediate; shows labradorescence
- Anorthite (CaAl₂Si₂O₈): Calcium end-member
Physical Properties
| Property | Value |
|---|---|
| Crystal system | Monoclinic (orthoclase) or Triclinic (plagioclase) |
| Hardness | 6-6.5 Mohs |
| Specific gravity | 2.55-2.76 |
| Refractive index | 1.518-1.588 |
| Birefringence | 0.005-0.010 |
| Cleavage | Perfect in two directions at ~90° |
Moonstone
Moonstone displays adularescence—a soft, billowy glow that appears to float
just below the surface.
Cause of Adularescence
- Alternating layers of orthoclase and albite
- Layers form during cooling (exsolution)
- Layer thickness determines colour: thin = blue; thicker = white
- Best effect requires ~0.1 μm layers
Moonstone Types
| Type | Feldspar | Adularescence | Body Colour |
|---|---|---|---|
| Classical moonstone | Orthoclase | Blue-white schiller | Colourless to grey |
| Rainbow moonstone | Labradorite | Multicoloured flashes | Translucent white |
| Peach moonstone | Orthoclase | White schiller | Peach to orange |
Quality Factors
- Schiller colour: Blue is most prized
- Intensity: Strong, visible adularescence
- Body colour: Colourless preferred
- Transparency: More transparent = higher value
- Centred effect: Schiller centred on dome
Sources
- Sri Lanka: Finest blue moonstone (historic)
- India: Large production; variable quality
- Myanmar: Some fine material
- Tanzania: Good quality moonstones
Blue Moonstone
Labradorite
Labradorite displays labradorescence—striking plays of colour including
blue, green, gold, and purple.
Cause of Labradorescence
- Light interference from lamellar twinning
- Twin lamellae of varying thickness
- Colours appear in patches or broad flashes
- Best seen on specific crystal planes
Spectrolite
Finnish labradorite showing full spectral colours:
- Vivid blues, greens, oranges, purples
- Displays most of the visible spectrum
- Premium prices for fine examples
- Discovered in Finland during WWII
Sources
| Origin | Characteristics |
|---|---|
| Finland | Spectrolite; full colour range |
| Madagascar | Good labradorescence; various colours |
| Canada (Labrador) | Type locality; blue-green common |
| Mexico | Fire obsidian (different material) |
Sunstone
Sunstone displays aventurescence—a glittery, metallic reflection from
included platelets.
Cause of Aventurescence
Reflective inclusions cause the sparkle:
- Oregon sunstone: Copper platelets (unique)
- Indian sunstone: Hematite or goethite platelets
- Norwegian sunstone: Hematite platelets
Oregon Sunstone
Unique copper-bearing feldspar:
- Only source of copper-included feldspar
- Can show strong red/green dichroism
- Some stones are transparent without schiller
- Bicolour and tricolour specimens exist
- Premium for strong copper "schiller"
Quality Factors
- Evenness: Well-distributed sparkle
- Intensity: Strong metallic effect
- Body colour: Can add value (Oregon reds)
- Transparency: Some prefer transparency; others prefer schiller
Amazonite
Blue-green microcline feldspar:
- Colour cause: Lead and water in structure
- Colour range: Blue-green to green
- Character: Opaque to translucent
- Uses: Cabochons, carvings, beads
- Sources: Brazil, Russia (Urals), Colorado, Madagascar
Cleavage Concerns
Identification Summary
Key features for feldspar identification:
- RI: 1.518-1.588 (variable by species)
- SG: 2.55-2.76
- Birefringence: 0.005-0.010
- Cleavage: Two directions at ~90°
- Phenomena: Diagnostic (adularescence, labradorescence)
- Twinning: Common; visible under polariscope