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 is 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.