Feldspar Group
Feldspar group gems including moonstone, labradorite, sunstone, amazonite, and orthoclase with properties, phenomena, and identification.
Introduction
Feldspars are framework aluminosilicates — the most abundant mineral group in Earth's
crust — crystallising in the monoclinic (orthoclase) or triclinic (plagioclase) system.
Gem members span the alkali series (potassium orthoclase and microcline) and the
sodium-calcium plagioclase series (albite through labradorite). All feldspars share
hardness 6–6.5 Mohs, SG 2.55–2.76, RI 1.518–1.588, birefringence 0.005–0.010, and
perfect cleavage in two directions — the cleavage being a significant durability concern
in jewellery settings. [1] The group's chief gem significance lies in its optical
phenomena: orthoclase moonstone shows adularescence from alternating albite and orthoclase
lamellae that diffract light [2], labradorite shows
labradorescence from lamellar twinning, and Oregon sunstone shows aventurescence from
unique copper platelets. Fine blue moonstone from Sri Lanka — colourless to pale grey
body with a floating blue schiller — is the most sought feldpar variety, commanding
significant premiums over cloudy or white material; Finnish spectrolite showing the full
visible spectrum is the finest labradorite. [3]
Feldspar Classification
Alkali Feldspars
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
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 [1]
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 [3]
- 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
References
- ↑ 1. Read, P. (2008). Gemmology (3rd ed.). Butterworth-Heinemann. ISBN: 978-0-7506-6449-3. DOI: 10.4324/9780080507224.
- ↑ 2. Fritsch, E.; Rossman, G. (1988). An Update on Color in Gems. Part 3: Colors Caused By Band Gaps and Physical Phenomena. Gems & Gemology, 24(2), 81–102. DOI: 10.5741/gems.24.2.81.
- ↑ 3. Fritsch, E.; Rossman, G. (1987). An Update on Color in Gems. Part 1: Introduction and Colors Caused by Dispersed Metal Ions. Gems & Gemology, 23(3), 126–139. DOI: 10.5741/gems.23.3.126.