Moonstone is a feldspar gem known for a soft light that seems to float just under its surface and glide across the stone as it moves. Gemologists call this effect adularescence. The finest moonstones are nearly transparent with a blue sheen, and most come from Sri Lanka and India. Moonstone shares the June birthstone spot with pearl and alexandrite.
What moonstone is
Feldspars are the most common minerals in the Earth's crust, and only a few of them reach gem quality. Classic moonstone is orthoclase, a potassium feldspar, intergrown with thin layers of albite, a sodium feldspar. Some gem references also use the name for related feldspars that show the same glow, such as sanidine and some oligoclase.
The two feldspars mix freely when the rock is very hot. As it cools, they separate into alternating microscopic layers, a process called exsolution. The layers are stacked like pages in a book. Light entering the stone scatters off these layers and comes back as a diffuse glow.
What causes the glow
The thickness of the layers controls the color of the sheen. Very thin layers scatter mostly short wavelengths, so the stone shows a blue glow. Thicker layers scatter all wavelengths more evenly and give a white or silvery sheen. That is why blue-sheen moonstone is rarer and prized above white-sheen material.
Adularescence takes its name from adularia, a variety of orthoclase named after the Adula massif in Switzerland. The page on gemstone optics compares it with related effects such as labradorescence and chatoyancy.
Some moonstones contain parallel needle-like inclusions as well, and these can produce a cat's eye or a four-rayed star when cut as a cabochon. Star and cat's eye moonstones come mostly from India.
Colors and varieties
Moonstone's body color ranges from colorless through gray, white, peach, champagne, brown and green. The trade sorts it into a few groups:
- Blue moonstone has a colorless, near-transparent body and a strong blue sheen. Sri Lanka has been the classic source, with the Meetiyagoda area famous for it.
- White moonstone has a translucent body and a milky white glow. It is the most common type.
- Peach, gray and champagne moonstones from India have colored bodies with a white or silvery sheen.
- Cat's eye and star moonstone show a thin band or star of light on top of the glow.
Rainbow moonstone is labradorite
The stone sold as rainbow moonstone is a different feldspar. It is a nearly colorless labradorite, a plagioclase feldspar, that shows a blue sheen with flashes of other spectral colors. Mineralogically it belongs with labradorite, and its optical effect is closer to labradorescence. The trade name is widely used and accepted, and some gemologists prefer to call it rainbow labradorite or white labradorite. It often shows thin black inclusions of tourmaline or other minerals, which ordinary moonstone does not.
Both stones have similar hardness and cleavage, so they need the same care.
Sources
Sri Lanka has supplied moonstone for centuries, much of it from weathered deposits and gem gravels. India produces large amounts of colored and rainbow moonstone, and the gems cut there make up a large share of commercial material. Myanmar, Madagascar, Tanzania, Brazil and the United States have also produced moonstone.
Moonstone forms in igneous rocks such as granite pegmatites and in some metamorphic rocks. The slow cooling that allows feldspar layers to separate is the reason it is tied to these deep-seated rocks. The igneous rocks and gemstones page explains how pegmatites form.
How moonstone is cut
Cutters almost always shape moonstone as a domed cabochon, because a curved surface concentrates the glow into a floating band. The orientation matters. The cutter must place the dome so the layered structure lies parallel to the base, which puts the sheen in the center of the stone. A poorly oriented stone shows its glow off to one side or only at a tilt.
A higher dome usually gives a sharper, more mobile glow. Moonstone is also carved into faces and figures, and transparent material is sometimes faceted, though faceting does not show adularescence well.
Judging quality
Value depends mostly on three things: the body's transparency, the color and strength of the sheen, and how centered the glow is. The finest stones have a clear body with a strong blue glow that covers the whole dome from any viewing angle. Visible inclusions, cracks or a cloudy body lower the value, as does a weak sheen.
Moonstone often shows tiny inclusions called centipedes. These are small stress cracks along cleavage directions that look like short lines with legs, and they help identify natural moonstone. The page on gemstone diaphaneity explains the terms used for transparency.
Imitations include milky glass, synthetic spinel made to look like moonstone, and opalescent plastic. Glass imitations often show a uniform glow with no movement and may contain gas bubbles.
Durability and care
Moonstone has a hardness of 6 to 6.5, which is softer than quartz dust. It also has perfect cleavage in two directions, so a sharp knock can split it. Rings take more abuse than pendants and earrings, so moonstone rings do best in protective bezel settings and with occasional care. The gemstone durability page compares wearability across gems.
Clean moonstone with warm soapy water and a soft brush. Avoid ultrasonic and steam cleaners, which can open cleavage cracks, and avoid sudden temperature changes.
Moonstone in history and tradition
A long-repeated tradition says the Romans believed moonstone formed from solid moonlight, and moonstone appears in Roman jewelry. Art Nouveau jewelers around 1900, René Lalique among them, used moonstone widely for its soft glow. In the United States, Florida named moonstone its state gem in 1970 to honor the moon landings launched from the state, even though moonstone does not occur naturally there.
Moonstone is one of the June birthstones. The birthstones by month page lists the others.
Frequently asked questions
Is moonstone good for an engagement ring?
It can work with care. Moonstone scratches more easily than sapphire or diamond and can split if hit, so a protective bezel and removing the ring for rough work help it last.
What is the most expensive type of moonstone?
Near-transparent stones with a strong blue sheen that covers the whole dome are the most prized. Large clean blue moonstones are rare.
Is rainbow moonstone real moonstone?
Rainbow moonstone is a natural gem, but mineralogically it is labradorite, a different feldspar from classic orthoclase moonstone.
Why does my moonstone look dull?
Scratches, a film of skin oil or a cabochon cut off-center can weaken the glow. Cleaning helps, and a jeweler can repolish a scratched stone.