Moonstone is not one of the stones we see faked often, but it is still worth checking. Real moonstone's sheen is soft, blue-white and directional. Opalite (glass) shows a warm orange glow when backlit and a cold blue glow in reflected light, plus round bubbles under a loupe. Moonstone scratches with quartz but not with a knife, and shows ~90° cleavage steps. A photograph gets you most of the way; the physical test settles it.
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A photograph cannot measure hardness, streak or density. Run the test below to be certain.
The test that settles it
Real moonstone's sheen is soft, blue-white and directional. Opalite (glass) shows a warm orange glow when backlit and a cold blue glow in reflected light, plus round bubbles under a loupe. Moonstone scratches with quartz but not with a knife, and shows ~90° cleavage steps.
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A photograph cannot measure hardness, streak or specific gravity. We tell you what the visual signs suggest and give you the test to be certain yourself.
Real moonstone has one optical effect and it is very specific. Light entering the stone hits alternating microscopic layers of orthoclase and albite that grew as the feldspar cooled and unmixed, and scatters back as a soft blue-white glow that appears to float a millimetre or two below the surface. Tilt the stone and the glow sweeps across it like a beam, brightest at one angle and gone at another.
That is adularescence. It is diffuse, billowy and close to monochrome, white through pale blue, or a warm peach in peach material. If what you are looking at is a spectrum of separate colours, a sparkle sitting on the surface, or a glitter that does not move as you tilt, it is not adularescence and the stone is not moonstone.
Opalite is glass, and it gives itself away with a torch
The most common substitute is opalite, a manufactured milky glass sold under names like sea opal or opalite moonstone. Backlight it with a phone torch held behind the stone: opalite glows a warm orange amber. Look at it again in reflected light against a dark background and it goes cold blue. That orange-to-blue flip is a Rayleigh-scattering signature of the glass, and real moonstone never does it, it stays white-blue in both.
Then take a 10x loupe to it. Glass carries round gas bubbles, swirl lines and a moulded seam near the drill hole or the edge of a cabochon. Moonstone has none of those. What it has instead is straight, parallel stress cracks, short and stacked, that collectors call centipede inclusions. They look like the legs of an insect running through the stone, and they are close to diagnostic.
Hardness and cleavage: two tests you can do at a kitchen table
Moonstone is a feldspar, formula (Na,K)AlSi3O8, hardness 6 to 6.5. That sits in a narrow useful window. A steel knife or a steel nail is about 5.5 and will not scratch it, while a quartz point at 7 will leave a mark. Glass is around 5.5 as well, so a substitute that scratches with the knife is not moonstone. Test on the back of the stone or on a hidden spot near the setting, never on the display face.
Feldspar also cleaves in two directions that meet at roughly 90 degrees. Look at the girdle or any chipped edge under the loupe and you may see flat, stepped surfaces that look like tiny staircases, reflecting all at once as you rotate the stone. Glass has no cleavage at all, it breaks in curved conchoidal shells with sharp lips.
Rainbow moonstone is a different mineral, and that matters
Most of what is sold as rainbow moonstone is white labradorite, a plagioclase feldspar, not the orthoclase that adularia moonstone comes from. It is a real stone honestly worth buying, and the flash is genuine, but it is a spectral flash of blue, gold and green, labradorescence, rather than the soft single-toned float of true moonstone. If you were charged a premium for classic Sri Lankan blue-sheen moonstone and received a rainbow flash, you were sold something else.
The other honest lookalikes are milky quartz, which is uniformly cloudy and has no moving glow, chalcedony, which is waxy and inert, and selenite, which is so soft your fingernail marks it. Selenite at hardness 2 is the fastest of all to rule out.
Sources, price and what an obvious bargain means
Good material comes from Sri Lanka, India, Madagascar, Myanmar, Tanzania and Virginia in the United States. Sri Lankan blue-sheen stones with a clear body and a strong beam are the expensive end. Indian material is more plentiful and more included, which is why it is cheap and why it is also where most honest tumbles come from. A large, perfectly clean, strongly glowing bead strand priced at a few pounds is glass, because the rough for that stone would cost more than the strand.
Care, once you know it is real
Those same two cleavage planes make moonstone brittle. It chips on impact and cracks with sudden temperature change, so keep it out of an ultrasonic cleaner and out of boiling water. Warm water, mild soap, a soft cloth. Store it away from quartz and topaz in a jewellery box, because anything harder than 6.5 will scratch it in a drawer.
Common questions
How can you tell if moonstone is real?
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Real moonstone's sheen is soft, blue-white and directional. Opalite (glass) shows a warm orange glow when backlit and a cold blue glow in reflected light, plus round bubbles under a loupe. Moonstone scratches with quartz but not with a knife, and shows ~90° cleavage steps. A photograph cannot measure hardness, streak or specific gravity, so treat a visual match as a strong hint and run the physical test to be certain.
Its formula is (Na,K)AlSi3O8, and it measures 6-6.5 on the Mohs scale. A hardness test against a known material is the cheapest way to rule out a substitute.
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