Adularescence
Adularescence in moonstone including cause, types of moonstone, quality factors, and distinction from labradorescence.
Introduction
Adularescence is the soft, billowy, blue to white glow that appears to float
just beneath the surface of moonstone, shifting gently as the stone is moved
under light. Named after Mount Adula in the Swiss Alps (a historic source of
fine material), it is the defining optical phenomenon of gem-quality moonstone
and wholly distinct from the polychromatic flashes of labradorescence.
The effect is caused by thin-film scattering within the feldspar: during slow
cooling, moonstone separates into alternating lamellae of orthoclase and albite
through a process called exsolution. When these layers measure approximately
60–150 nm, close to the wavelength of visible light, they selectively scatter
shorter wavelengths, producing the characteristic blue schiller. Thicker layers
(~200–300 nm) scatter broadly and yield a white glow; very thick layers produce
no visible effect. The finest blue adularescence, seen in Sri Lankan orthoclase
moonstone, owes its colour to exceptionally thin albite layers near 100 nm.
[1][2]
Cause
The physics behind adularescence:
Feldspar Structure
Types of Moonstone
| 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 |
| Grey moonstone | Orthoclase | Blue schiller | Grey body |
Rainbow Moonstone Note
Quality Factors
Assessing moonstone quality:
Schiller Quality
| Factor | Premium Quality |
|---|---|
| Schiller colour | Blue (most valued) |
| Intensity | Strong, visible adularescence |
| Centering | Schiller centred on dome |
| Coverage | Covers most of dome surface |
| Movement | Smooth, floating motion |
Body Quality
- Transparency: More transparent = higher value
- Body colour: Colourless to light grey preferred
- Clarity: Fewer inclusions better
- Cut: Well-proportioned cabochon
Sources
| Origin | Characteristics |
|---|---|
| Sri Lanka | Finest blue moonstone; historic source |
| India | Large production; variable quality |
| Myanmar | Some fine material |
| Tanzania | Good quality available |
| Madagascar | Various qualities |
Adularescence vs Labradorescence
Adularescence (Moonstone)
- Single-colour glow (blue or white)
- Soft, billowy appearance
- Appears to float below surface
- Caused by layer interference
- Orthoclase feldspar
Labradorescence (Labradorite)
- Multiple spectral colours
- Sharp, distinct flashes
- Appears on surface
- Caused by twinning interference
- Plagioclase feldspar
Cutting for Adularescence
Proper orientation is critical:
- Cut as cabochon (faceting destroys effect)
- Orient to show schiller through dome
- Appropriate dome height
- Symmetrical shape for even display
- Polish quality important for clarity
Care Considerations
References
- ↑ 1. Read, P. (2014). Gemmology (3rd ed.). Butterworth-Heinemann/Routledge. DOI: 10.4324/9780080507224.
- ↑ 2. Nassau, K. (2001). The Physics and Chemistry of Color (2nd ed.). Wiley-Interscience. ISBN: 978-0-471-39106-7.
- ↑ 3. Schumann, W. (2009). Gemstones of the World (4th ed.). Sterling. ISBN: 978-1-4027-6829-3.