Gem library
Moonstone
Orthoclase with a floating blue veil
Moonstone is an orthoclase whose interior diffuses a bluish veil that seems to float beneath the surface: adularescence. That glow arises from microscopic albite lamellae interleaved in the potassium feldspar, which scatter blue light.
- Hardness (Mohs)
- 6 – 6.5
- Density
- 2.55 – 2.63
- Crystal system
- Monoclinic
- Refractive index
- 1.518 – 1.526
- Cleavage
- Perfect on {001}, distinct on {010}
Mineralogical identity
- Family
- Feldspars — orthoclase / adularia
- Species
- Orthoclase
- Variety
- Adularia (moonstone)
- Chemical formula
- KAlSi₃O₈ with albite lamellae
- Crystal system
- Monoclinic
- Classification
- Semi-precious stone
- Rarity
- Common
- Indicative price
- €1 – 60/ct depending on sheen quality
Etymology
Descriptive: the sheen evokes moonlight. The mineralogical name adularia comes from the Adula massif in Switzerland.
Physical & optical properties
- Hardness (Mohs)
- 6 – 6.5
- Density
- 2.55 – 2.63
- Refractive index
- 1.518 – 1.526
- Birefringence
- 0.005 – 0.008
- Cleavage
- Perfect on {001}, distinct on {010}
- Fracture
- Splintery
- Lustre
- Vitreous to pearly, adularescent
- Diaphaneity
- Translucent to transparent
- UV fluorescence
- Weak
- Pleochroism
- None
- Colours
- white, colourless, blue sheen
Formation, geology & history
Adularescence explained
As the crystal cools slowly, orthoclase and albite — two feldspars of different composition — separate into alternating lamellae of submicron thickness. These layers preferentially scatter short wavelengths — blue — by Rayleigh scattering, the same process that makes the sky blue. The result is a soft, mobile glow that seems to come from within the stone and shifts as you tilt it. Unlike labradorescence, which is livelier and more colourful, adularescence is milky and diffuse. The finest moonstones are water-clear with an electric blue sheen perfectly centred: those come from Sri Lanka.
Trade vocabulary
This is a terminological minefield. Rainbow moonstone is a white labradorite, not an orthoclase. Indian rainbow, black and green «moonstones» are labradorites or other plagioclases. True moonstone, adularia, is a colourless to white orthoclase with a blue or white sheen. Sunstone is yet another feldspar, with hematite or copper platelets. And opal has nothing to do with any of it: its play of colour comes from silica spheres, not feldspar lamellae.
History and jewellery
The Romans believed it solidified from moonbeams, and Pliny reports that it changed with the lunar phases. Its heyday came with Art Nouveau: René Lalique, Louis Comfort Tiffany and Georges Fouquet used it lavishly, drawn to a milky glow that suited enamels and plant lines. Hardness 6 and a double cleavage call for thick cabochons and protective settings; moonstone rings worn daily almost always end up chipping at the edges.
How to recognise it
- The veil movesTilt the stone: the bluish glow must glide beneath the surface like a mobile mist. A fixed sheen indicates opalescent glass.
- Transparent groundFine moonstones are transparent or very translucent, with the veil floating inside. A milky opaque stone is of lesser quality or a different feldspar.
- Visible pearly cleavagesIn grazing light, fine flat shiny lines appear: the feldspar cleavage planes.
Imitations, treatments & confusions
- Opalescent glass (opalite, «synthetic moonstone»)Very common: milky glass with a uniform bluish sheen. No cleavage, bubbles under the loupe, immediate warmth, an over-regular sheen.
- White chalcedony or milky quartzNo mobile adularescence, only milky translucency. The higher hardness (7) is a clue.
- White labradorite sold as moonstoneNot a fake but a naming confusion; labradorite throws vivid coloured flashes, moonstone diffuses a soft milky glow.
Deposits & origins
Sri Lanka (the finest, transparent with intense blue sheen), India (grey, brown and green moonstones with strong adularescence), Burma, Madagascar, Tanzania, United States (Virginia), Norway
Care & precautions
- Warm soapy water and a very soft brush, brief rinsing. Dry at once.
- Mind impacts: the double cleavage makes it fragile despite hardness 6. Store it apart.
- No ultrasonic cleaning, no steam: vibration and sudden heat can start a cleavage.
- Sunlight does not harm it, but avoid thermal shock (hot water on a cold stone).
Traditions & crystal healing
- Chakras
- Sacral · crown
- Zodiac signs
- Cancer · Pisces · Libra
- Element
- Water
- Month
- June
A stone of femininity and cycles in tradition, linked to the moon and to inner tides. It is credited with soothing changeable emotions, supporting periods of transition and connecting to intuition. The Romans already wore it as a dream amulet, placed under the pillow.
The virtues described here belong to tradition and cultural heritage. They are in no way medical advice and never replace a consultation.
Frequently asked questions
Moonstone or opalite?
Opalite is glass: uniform bluish sheen, bubbles, warmth in the hand. Moonstone is a feldspar: mobile veil, pearly cleavage planes, lasting coolness.
Is rainbow moonstone a moonstone?
Mineralogically, no: it is a white labradorite. The name is purely commercial. True moonstone is an orthoclase with a milky blue sheen.
Can it be worn as an everyday ring?
Not ideally. Hardness 6 with double cleavage: the edges chip quickly. Prefer a thick cabochon in a closed setting, or a pendant.