Moonstone

Moonstone is bought for a glow that is not really in the stone. Tilt a good one and a soft blue light rolls across the dome, appearing to float somewhere beneath the surface rather than sitting on it. The effect is called adularescence, and moonstone is the material it is named for and still the one that shows it best.

It is also a stone where the trade names are actively misleading. Rainbow moonstone is not moonstone in the strict sense, and a great deal of what is sold as moonstone is a different feldspar entirely. None of that is necessarily dishonest, but it does mean the word alone tells you very little about what you are buying.

This page covers what produces the sheen, how to judge one, the naming problem, the cleavage that makes it a delicate ring stone, and what the good material actually costs.

What makes the glow

Moonstone is a feldspar that separated as it cooled into alternating microscopic layers of two feldspars, usually orthoclase and albite, with slightly different refractive indices. Light entering the stone scatters off those layers and comes back diffused and shifted towards blue, which is why the glow appears to hover below the surface.

The layers have to be the right thickness. Too thick and the scattering produces a whitish rather than a blue sheen; too thin and there is barely any effect. Blue adularescence is the scarcer outcome, which is the physical reason blue moonstone costs several times what white does.

Because the effect depends on the orientation of those layers, the cutter has to align the dome so the sheen sits centred and rises when the stone is viewed face up. A stone cut off-axis shows its glow sliding off the edge, and no amount of polishing afterwards can move it back.

Judging a moonstone

Four things, and the first two matter far more than the rest:

The combination people pay for is a transparent colourless body with a strong blue sheen, which is scarce and is what fine Sri Lankan material is known for. A translucent milky white body with a white sheen is the everyday product, and there is a very large price gap between the two even though photographs of them can look similar.

Rainbow moonstone is not moonstone

What the trade calls rainbow moonstone is white labradorite, a different feldspar. It shows broad flashes of blue, green, gold and violet across a semi-transparent white body, and it is genuinely attractive, but the optical effect is labradorescence rather than adularescence and the material is more common and less expensive.

The name is now so entrenched that objecting to it is futile, and most sellers using it are not trying to deceive anyone. Simply know that rainbow moonstone and blue moonstone are two different stones at two different prices, and that a listing offering large rainbow moonstone cheaply is not offering you a bargain on the blue material.

Where it comes from

Sri Lanka is the classic source for fine blue material with a colourless transparent body, and the standard is still set there. India produces a great deal of moonstone including most of the white labradorite sold as rainbow, Myanmar has historically produced excellent blue material, and Madagascar and Tanzania both contribute to current supply.

Origin is not a meaningful price factor. Nobody pays a premium for a country here; they pay for the sheen. Judge the stone in the hand or on video and ignore the passport.

Hardness, cleavage and everyday wear

Moonstone is 6 to 6.5 on the Mohs scale, and more importantly it has two directions of perfect cleavage meeting at close to a right angle. That combination makes it genuinely fragile: it will scratch from ordinary dust, and a knock in the wrong direction can split it rather than scuff it.

It is a lovely stone and a poor choice for a ring worn every day without protection. If it is going into a ring, our guide to settings is the place to start, and a bezel that covers the girdle entirely is close to mandatory. In earrings and pendants none of this matters much, and the stone will last indefinitely.

Keep it away from ultrasonic and steam cleaners, both of which can propagate an existing cleavage crack into a break. Warm water and a soft cloth are enough.

What it costs

Almost entirely a function of sheen colour and body transparency. In dollars per carat:

Size matters less than it does in faceted stones, because what you are buying is an effect rather than a volume of colour. A small stone with a vivid centred blue sheen is worth more than a large dull one, and the market prices it that way.

Meaning and symbolism

Moonstone is a June birthstone alongside pearl and alexandrite, and the June page covers all three. The symbolism is unusually consistent and follows directly from the name: intuition, cycles, and safe travel, particularly travel over water.

In Indian tradition it is a sacred stone associated with the moon and is a traditional gift between lovers, which combined with the June association makes it a recurring choice for engagement and anniversary pieces. Roman writers believed it was moonlight solidified, which is a better guess than most ancient gem theories.

If you are choosing by tradition rather than by month, our astrology and gemstones guide sets out the zodiac schemes, which assign stones on a different basis.

Designing around a moonstone

• Bezel it. Two cleavage directions and a soft surface make an exposed girdle the obvious failure point. Our settings guide covers the alternatives.
• Check the sheen is centred before setting. The orientation is fixed by the cutter, and a stone that glows off-axis will glow off-axis forever.
• White metal suits the blue. Silver, white gold and platinum all keep the sheen cool; yellow gold warms it and can fight the blue. Our metals guide covers the choice.
• Keep the setting quiet. The glow is subtle and a busy halo of brilliants simply outshouts it.
• Consider earrings. As with opal, this stone lasts far better where it meets nothing, and the money buys a better example.

A short buying checklist

• Ask for video with the stone tilting slowly, since a still photograph cannot show adularescence.
• Establish whether it is moonstone or white labradorite sold as rainbow moonstone.
• Check the sheen is blue rather than white, and that it rises from the centre face up.
• Look for internal cracks, which are common in feldspar and are a durability problem as well as a clarity one.
• Ask whether the body is transparent or milky, since that is half the price difference.
• For anything substantial, ask for a report from an independent laboratory naming the feldspar species.

Frequently asked questions

Is rainbow moonstone real moonstone? Not in the strict sense. It is white labradorite, a related feldspar showing a different optical effect. It is a genuine gemstone, just not the same one, and it costs considerably less.

Why is blue moonstone so much more expensive? Because the internal layering that produces a blue sheen has to fall in a narrow thickness range. White sheen is the common outcome and blue is the scarce one.

Is moonstone strong enough for an engagement ring? Not really, without protection. At 6 to 6.5 with two perfect cleavage directions it scratches and can split. In a bezel, worn thoughtfully, it works; as an unprotected daily solitaire it does not.

Is moonstone treated? Usually not. It is occasionally coated or backed with foil to strengthen the apparent sheen, and dyed material appears at the very low end, but most moonstone on the market is untreated.

Why does mine look grey rather than blue? Either the sheen is white rather than blue, or you are viewing it off-axis. Tilt it slowly under a single light and see whether a blue glow rises anywhere.

What is the difference between moonstone and opalite? Opalite is manufactured glass. It has a milky glow that superficially resembles moonstone and it costs almost nothing. If a large flawless glowing stone is very cheap, this is usually what it is.

Can I wear it in the shower? Better not. It is not water-soluble, but soap builds up in surface cracks and thermal shock is a real risk in a stone with cleavage.

How do I clean it? Warm water, mild soap and a soft cloth. No ultrasonic, no steam, no solvents.

To see how it sits against the rest of the coloured stones, the semi-precious stones guide compares the series side by side, and we are happy to answer questions directly (opens in new tab).