May Birthstone: Emerald, and Why It Needs Different Care

By Lisa Chen, Founder · July 16, 2026

Short answer: May’s birthstone is emerald — the green variety of beryl, which the U.S. Geological Survey places at 7.5 to 8 on the Mohs hardness scale. That is harder than amethyst and harder than jade, and it is the most misleading number on the birthstone calendar. Emerald is hard. Emerald is not durable. Those are two different properties, and the gap is why May’s stone needs care rules almost no other stone needs.

What emerald actually is

Emerald is not its own mineral species. As GIA defines it, emerald is the green to bluish green variety of beryl — the species that also gives us aquamarine and morganite. The USGS lists the family under one heading: beryl, a beryllium aluminum silicate, hardness 7.5–8, with emerald as the intense green member. The color comes from traces of chromium, vanadium and iron; GIA notes that stones too light to earn the name are called green beryl and are worth considerably less.

Why hardness is the wrong number to trust

Durability is not one property. GIA breaks it into three: hardness, resistance to scratches; toughness, resistance to breaking; stability, resistance to chemicals and temperature. Mohs measures only the first, and emerald scores well there and poorly on the other two. The reason sits inside the crystal. GIA is blunt: almost all emeralds have significant fractures, and partly because of them, emeralds are more brittle than a gem like corundum — vulnerable during cutting, polishing and setting, or, in GIA’s phrase, even during careless daily wear. The emerald cut is named for this stone because faceting off the vulnerable corners gives prongs a safer place to grip — a cut invented around fragility.

Those fractures come with company. Emeralds typically contain inclusions visible to the naked eye; the trade accepts them rather than calling them flaws, and eye-clean stones are rare. They look mossy or garden-like, which is why the trade calls them jardin, French for garden — an emerald’s interior weather is its identity and its weakness.

Put the two side by side and the picture inverts. Hardness figures are the USGS’s or GIA’s; toughness ratings are GIA’s wording.

Stone	Mohs hardness	Toughness (GIA)	What that means in practice
Nephrite jade	6–6.5 (GIA)	Exceptional	Scratches more easily, but very resistant to breaking or chipping
Amethyst (quartz)	7 (USGS)	Good	Suitable for all jewelry types, including rings
Emerald (beryl)	7.5–8 (USGS)	Fair to good	Harder than both above, yet needs more care in wearing than ruby or sapphire
Ruby and sapphire (corundum)	9 (USGS)	Excellent	Excellent toughness and no cleavage — a great choice for daily wear

Jade is the proof: GIA rates nephrite 6 to 6.5, clearly softer than emerald, yet calls its toughness exceptional. Knock a jade bangle and a set emerald against the same doorframe and the softer stone walks away. Emerald ranks third of four here on hardness, last on toughness: difficult to scratch, easy to break.

The treatment picture, said plainly

Nearly every emerald has been worked on. GIA’s care guide reports that some estimates state 90 percent or more of emeralds are fracture-filled. Per GIA’s treatment guide:

• Fracture filling is the main event. Surface-reaching fractures are filled with oils, waxes and artificial resins to reduce their visibility and improve the stone’s apparent clarity. The fractures are still there. They are just harder to see.
• The volume varies enormously. GIA says filler ranges from insignificant to major amounts, with varying stability. Its laboratory classifies the level of enhancement as minor, moderate or significant — a measure of treatment, not a clarity grade. Paler fractured stones are sometimes dyed green too.
• It is not permanent — the key fact. Much depends on the filler: glasses outlast resins, oils and waxes, and air-pressure changes, heat or chemicals can alter or remove it. An oiled emerald can lose its oil, and the jardin returns. Re-treatment over the years is normal.

That impermanence puts it inside the FTC’s rule: failing to disclose a gemstone treatment is deceptive when it is not permanent, creates special care requirements, or significantly affects value. Emerald filling can tick all three. My honest read: a filled emerald is not a lesser stone, but a seller who dodges “how was this one treated?” is a lesser seller.

The care rules, and why each one exists

Every rule follows from one fact: there is a foreign substance in the cracks of your stone, and most things that clean jewelry are good at getting it out.

• Never an ultrasonic cleaner. GIA’s May guide says never: the vibrations and heat can make the filler sweat out of the fractures, and can weaken an already-fractured stone.
• No steam, no heat. Hot steam can cause oil or unhardened resin to sweat out, and heat can damage emeralds, GIA says, especially by extending existing fractures.
• No chemicals or solvents. Light and chemicals make the oils, resins and polymers deteriorate — acetone, alcohol, any jewelry dip.
• Two rules nobody hears. GIA flags air-pressure changes, such as an airline cabin, and notes filled emeralds can be damaged by hot dishwater.
• What you actually do: warm, soapy water with gentle scrubbing. GIA calls it the safest way, and there is no step two. Store the piece apart — corundum at 9 and diamond at 10 will scratch beryl in a shared pouch.

If a hard-wearing everyday piece brought you here rather than the stone, our necklaces and rings are worth a look.

Lab-grown emerald, and the green stones that are not emerald at all

These two get lumped together constantly, and they are opposites. Lab-grown emerald is real emerald. GIA defines a synthetic gem material as one made in a laboratory sharing virtually all the chemical, optical and physical characteristics of its natural counterpart. U.S. rules make it precise: “laboratory-grown emerald” may only be used if the product has essentially the same optical, physical and chemical properties as the mined stone, with the qualifier immediately before the word and equally conspicuous. The same rule makes the unqualified word “emerald” deceptive for anything not mined, and says “faux” is not adequate disclosure.

A simulant is a different material wearing the color. GIA defines a simulant as something that looks like another gem and substitutes for it while having very different chemical composition, crystal structure, and optical and physical properties. Cubic zirconia comes in almost any color, and GIA notes colored CZ makes good imitations of green gemstones; glass comes in virtually any color; synthetic spinel, GIA says, is a common choice for imitation birthstone jewelry specifically. Labeled honestly, none of that is a scam; it is simply not beryl. So emerald and lab-grown emerald mean beryl; simulated, faux and emerald-colored mean something else.

What emerald has traditionally meant

Emerald carries as much folklore as any stone on the calendar, and I want to report it carefully, because plenty of sellers do not. What follows is cultural history, not a claim that any of it works. GIA traces the name to the ancient Greek word for green; Pliny the Elder gave his first-century verdict in his Natural History: “nothing greens greener.” The first known mines were Egyptian, from at least 330 BC, and the Incas used emeralds for 500 years before sixteenth-century Spanish explorers shipped them home. Legend had it that an emerald under the tongue let you see the future or revealed the truth of a lover’s oath; it was once believed to cure cholera and malaria. Today it is the gem for the twentieth and thirty-fifth wedding anniversaries.

That is a magnificent inheritance and an honest reason to love the stone. But it is culture, not chemistry: emerald will not sharpen your wits, cure an illness, or move your luck, health or wealth, whatever a description implies. The list is younger than people assume — Jewelers of America, which describes itself as the originator of the U.S. birthstone list, dates it to 1912. May is a tidy month: one stone, no later additions. A century-old convention is a fine thing to enjoy without mistaking it for destiny.

Frequently asked questions

What is the May birthstone?

Emerald, the green to bluish green variety of beryl. May is one of the simplest months on the modern American list: a single stone, no later additions. Jewelers of America, the list’s self-described originator, dates it to 1912.

Is emerald hard enough for an everyday ring?

Hardness is not the problem; toughness is. The USGS puts beryl at 7.5 to 8 on the Mohs scale, harder than quartz or jade. But GIA rates emerald’s toughness only fair to good and says it needs more care in wearing than ruby or sapphire, because almost all emeralds contain significant fractures that make them more brittle than corundum. It can be worn daily with a protective setting and some care.

Are most emeralds treated?

Almost certainly. GIA reports estimates that 90 percent or more of emeralds are fracture-filled, using oils, waxes or artificial resins to hide surface-reaching fractures and improve apparent clarity. U.S. rules require a seller to disclose a treatment that is impermanent, creates special care requirements, or significantly affects value — and emerald filling can do all three.

Can you clean an emerald in an ultrasonic cleaner?

No. GIA says never: the vibrations and heat can make the filler sweat out of the fractures, and can weaken an already-fractured stone. Steam is out for the same reason, along with heat, chemicals and solvents. Warm soapy water and a soft brush is the safest method.

Is lab-grown emerald a real emerald?

Yes. Lab-grown emerald is synthetic beryl — GIA describes synthetic gem materials as sharing virtually all the properties of the natural stone, and U.S. rules only permit “laboratory-grown emerald” when that is true. A simulant such as green cubic zirconia or glass only resembles emerald, and must be labeled “imitation” or “simulated.”

The single rule for May’s stone: judge an emerald by its toughness, not its Mohs number. For more on how durability varies across the calendar, see our guides on the October birthstone and the April birthstone.

If you want to wear May’s stone rather than read about it, our Personalized Cross & Birthstone Necklace is made in a May version — 18k gold plating over a 316L stainless steel base, built for daily wear. The stones are cubic zirconia, not mined gems, and the product page says so.