A pearl and a moonstone can sit side by side and look like the same stone. Both glow with a soft, low light that seems to come from somewhere inside. People mix them up constantly. But they are not even close to the same thing: one is made by an animal, the other is made by the earth, and once you know where each one's light comes from, you will never confuse them again.

I set both. Pearls are one of the two stones in nearly every piece I make, and moonstone has carried whole collections. Here is how I tell them apart at the bench, and how you can tell them apart in your hand.

At a glance

The glow is the giveaway

If you only learn one thing, learn this: look at where the light lives.

On a pearl, the light sits on the surface. That soft, satiny shine is called lustre, and it is just light bouncing off the smooth, layered nacre the mollusk builds up. On a good pearl you may also see orient, the faint rainbow play that seems to dance on or just beneath the surface, like the colors in a film of oil on water. Lustre is the sheen; orient is the rainbow. Together they make a pearl's glow, and all of it reads as warm and solid, because no light passes through.

On a moonstone, the light comes from inside. That floating blue-white glow is called adularescence, and it drifts across the stone as you tilt it. It happens because moonstone is built from microscopically thin layers, and light scatters between them, so the glow looks like it is hovering just under the surface rather than sitting on top. No pearl does this. The moment you see light move as you tilt the stone, you are holding a moonstone.

What each one actually is

A pearl is organic. It is the only gem made by a living creature. A mollusk coats an irritant inside its shell with layer after layer of nacre, the same material that lines the shell, until a pearl forms. That is why no two pearls are ever truly identical, and why pearls are soft: nacre is delicate.

A moonstone is a mineral. It belongs to the feldspar family, the most common mineral group in the earth's crust, and it forms as two kinds of feldspar separate into thin, alternating layers as the stone cools. Those layers are what scatter the light into that blue glow. It is harder than a pearl, but feldspar cleaves, meaning it can split cleanly along its internal planes if it takes a sharp knock.

Durability and care, from the bench

This is where setting them teaches you the difference fast. A pearl is soft and porous, so it has to be protected from heat, from acids, even from perfume and hairspray. A moonstone has a tougher surface but it is brittle, so the danger is a sharp impact rather than a scratch.

For the wearer, the rule of thumb is the same for both, for opposite reasons. Pearls go on last and come off first so they never meet perfume or lotion. Moonstones can take daily wear but should be kept from hard knocks against other jewelry. To clean either one, skip the ultrasonic cleaner and skip heat. Neither stone tolerates them, and honestly I would not put any of my pieces through them. Warm water, a soft toothbrush, and a drop of Dawn lift away the oils and dust that dull a piece, and the polishing cloth tucked into every order brings the metal back to life.

Which one is for you

Here is the happy accident: both pearl and moonstone are June birthstones, so whichever speaks to you, you are covered.

Reach for a pearl if you love a warm, classic, dressed-up glow, the kind of light that has meant elegance for centuries. Reach for a moonstone if you are drawn to something cooler, more mysterious, a little otherworldly, with that blue light that moves when you do. One is the moon's reflection on calm water; the other is the moon itself.

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