Adularia Identification Guide
A field guide to adularia, a low-temperature potassium feldspar prized for moonstone sheen, covering crystal form, cleavage, and look-alikes.
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What Adularia Looks Like
Adularia is a low-temperature variety of potassium feldspar (orthoclase/microcline composition, KAlSi3O8) that crystallizes in clear hydrothermal veins. When it shows a floating blue-white sheen it is the gem moonstone (adularescence).
- Color: colorless, white, milky, or pale cream; gem moonstone shows a billowy blue or silvery-white sheen.
- Luster: vitreous to pearly on cleavage surfaces.
- Transparency: transparent to translucent.
- Habit: distinctive pseudo-orthorhombic, almost diamond-shaped (rhomb-like) crystals formed by combinations of prism and pinacoid faces; occurs lining vugs and Alpine-type fissures.
Step-by-Step Field ID Checklist
- Look for the sheen. Tilt the stone in light; a soft blue-to-white glow that moves (adularescence) points to adularia/moonstone.
- Check crystal shape. Adularia's pseudo-rhombic, wedge-shaped clear crystals on vein walls are characteristic.
- Test hardness. Mohs 6–6.5; it scratches glass but is scratched by quartz and a hard steel file.
- Find the two cleavages. Feldspar has two good cleavages meeting at nearly 90 degrees; look for blocky stepped surfaces.
- Check streak. White.
- Look at the setting. Alpine fissure or low-temperature hydrothermal vein, often with quartz, chlorite, and zeolites.
Key Diagnostic Tests
- Hardness: 6–6.5.
- Cleavage: two directions at ~90 degrees (perfect and good) — the feldspar signature.
- Specific gravity: ~2.55–2.6 (light).
- Streak: white.
- Adularescence: the schiller is caused by light scattering off microscopic lamellar intergrowths.
- Acid: none; not magnetic.
Common Look-Alikes and How to Tell Them Apart
- Plagioclase moonstone / albite: also can show schiller, but plagioclase commonly shows fine twin striations on cleavage faces (albite twinning) that K-feldspar adularia lacks.
- Quartz (rock crystal): harder (7), shows no 90-degree cleavage, and gives no moonstone sheen; quartz forms six-sided prisms with conchoidal fracture.
- Milky chalcedony / opal imitations: lack feldspar cleavage; opal shows play-of-color rather than the directional blue adularescence.
- Glass/synthetic 'opalite': no cleavage, often full of bubbles, and the sheen looks uniform rather than billowing.
- Labradorite: shows flashy spectral colors (labradorescence) rather than the soft single blue-white glow of adularia.
Where Adularia Is Found
Adularia is the classic mineral of Alpine-type clefts in the European Alps (the name comes from the Adula massif/Mt. Adular, Switzerland). Fine crystals come from Switzerland, Austria, and Italy. Gem moonstone adularia is famous from Sri Lanka and southern India, with additional material from Myanmar and Madagascar. Look for clear, sheeny K-feldspar crystals in low-temperature hydrothermal veins and fissures alongside quartz, chlorite, and zeolites.
Frequently asked questions
What is adularia?
Adularia is a low-temperature variety of potassium feldspar that crystallizes in hydrothermal veins and Alpine fissures; the gem version that shows a blue-white sheen is called moonstone.
How can you tell if it is real adularia?
Check for hardness 6–6.5, two cleavages meeting near 90 degrees, a white streak, low density, and a soft moving blue-white sheen; quartz look-alikes are harder and lack the right-angle cleavage.
Is adularia the same as moonstone?
Gem-quality adularia that displays adularescence (the floating blue sheen) is sold as moonstone, so all such moonstone is adularia, but plenty of plain adularia shows no sheen.
Adularia vs labradorite — what is the difference?
Adularia (a potassium feldspar) shows a soft single blue-white glow, while labradorite (a plagioclase feldspar) flashes multiple spectral colors known as labradorescence.
Where does the name adularia come from?
It is named after the Adula mountain group in the Swiss Alps, where fine crystals occur in Alpine-type clefts.