August Birthstone: Peridot, Spinel and Sardonyx

By Lisa Chen, Founder · July 12, 2026

Short answer: August has three birthstones — peridot, the main modern stone, plus spinel and sardonyx. Peridot is the yellow-green gem most people mean, sardonyx is the older traditional choice, and spinel is the newest addition. This guide covers what each stone is, how hard it is on the Mohs scale, how it holds up to everyday wear, whether it is treated, and how to care for it.

The three birthstones of August

Most months have a single birthstone. August has three, which puts it in rare company alongside June and December. The oldest, traditional August stone is sardonyx, a banded gem prized since antiquity. When the modern birthstone list took shape in the early twentieth century, peridot was included and became the month’s primary stone. Then in 2016 the American Gem Trade Association and Jewelers of America added spinel as a third option, according to the American Gem Society. So all three are “correct” — peridot is the default you will see on a chart, and the other two are legitimate alternatives with their own long histories.

Peridot: August’s green headliner

Peridot is the gem-quality form of olivine, a magnesium-iron silicate with the formula (Mg,Fe)₂SiO₄. What makes it unusual among colored stones is that its color is idiochromatic: the green comes from iron that is part of the mineral’s own chemistry, not from a trace impurity added on top. That is why peridot only ever occurs in green, ranging from a soft yellow-green to a deep olive, with a lively grass green the most prized tone.

On hardness, Jewelers of America places peridot at 6.5 to 7 on the Mohs scale. The United States Geological Survey’s Mohs hardness reference chart lists peridot (olivine) at 7, right at the top of that range and level with quartz — a number that matters more than it looks, as the durability section explains.

Peridot is also one of the oldest gems in continuous use, mined on Zabargad island in the Red Sea off Egypt for well over three thousand years; historians have long suspected some of Cleopatra’s famed “emeralds” were actually peridot. Today it comes mainly from Arizona’s San Carlos Apache Reservation, Pakistan, and Myanmar, and it is one of the few gems also found in meteorites, as gem-quality olivine inside pallasite fragments.

Spinel: the gem long mistaken for ruby

Spinel is magnesium aluminum oxide, MgAl₂O₄, while ruby is aluminum oxide, the mineral corundum — two genuinely different minerals, which is why spinel earned the nickname “the great impostor.” For centuries large red spinels were set into crowns and called rubies. The most famous example, the Black Prince’s Ruby, a roughly 170-carat red stone in the front of Britain’s Imperial State Crown, is actually a spinel; so is the Timur Ruby in the same collection. The two were only told apart once gemologists could measure them properly.

Spinel comes in red, pink, blue, and purple, with fine red and cobalt-blue stones the most sought after. Its practical virtue is durability: the USGS chart puts spinel at 8 on the Mohs scale, harder than every other August stone and tough enough to shrug off most everyday knocks.

Sardonyx: the original August stone

Sardonyx is the traditional August birthstone and the one most people have never handled. It is a banded variety of chalcedony, which is a microcrystalline form of quartz. The name is a portmanteau of its two ingredients: sard, the brownish-red to orange layers, and onyx, the white or black layers. Those parallel bands are the whole appeal. Because a carver can cut down through a pale layer to reveal a dark one beneath, sardonyx was the classic material for cameos and intaglios, and Roman seal rings were often cut from it because hot sealing wax does not stick to the polished stone.

As a quartz-family stone, sardonyx sits at roughly 6.5 to 7 on the Mohs scale, the same neighborhood as agate and onyx — durable enough for daily jewelry, with one caveat covered under treatments below: much of the banded chalcedony on the market has been dyed.

How the three compare

Here is the quick side-by-side. Every hardness figure below is from the USGS Mohs chart or Jewelers of America, not an estimate.

Stone	What it is	Mohs hardness	Typical color	Usual treatment
Peridot	Gem olivine, (Mg,Fe)₂SiO₄	6.5–7	Yellow-green to olive	None; no lab-grown, imitations are glass or CZ
Spinel	Magnesium aluminum oxide, MgAl₂O₄	8	Red, pink, blue, purple	Natural usually untreated; flame-fusion synthetic is common
Sardonyx	Banded chalcedony (sard + onyx)	6.5–7	Reddish-brown with white bands	Frequently dyed or heat-treated

What Mohs hardness means for everyday wear

Hardness on the Mohs scale measures scratch resistance, and the benchmark to remember is quartz at 7. Ordinary household dust contains fine quartz particles, so anything softer than quartz slowly picks up surface scratches from everyday handling. Spinel at 8 shrugs that off for years; peridot and sardonyx at 6.5 to 7 sit right at that dust line. It does not make them fragile, but it is why the softer two are happiest in earrings and pendants, and why spinel is the one to choose for a ring you wear all day.

Shopping for an August birthday and want something that survives daily life? Our rings collection is built around gold-plated stainless steel that holds up in the shower and the gym. One honesty note before you browse: those pieces set cubic zirconia and other synthetic stones for color, not mined peridot, spinel, or sardonyx — which keeps them affordable and durable, and is exactly the kind of thing a seller should say up front.

Treatments and imitations, stated plainly

This is where an honest guide earns its keep: “August birthstone” jewelry spans mined gems to inexpensive look-alikes.

• Peridot. Natural peridot is essentially sold untreated, and there is no commercial lab-grown peridot. Jewelers of America notes that peridot imitations are glass, cubic zirconia, or synthetic corundum, all of which must be disclosed. So if a “peridot” is suspiciously cheap and flawlessly clean, ask exactly what it is.
• Spinel. Fine natural spinel is usually untreated, but flame-fusion synthetic spinel has been mass-produced since the 1920s and is very common in class rings, costume pieces, and budget birthstone jewelry. Synthetic is not a dirty word; it just needs to be labeled as such.
• Sardonyx. A large share of banded chalcedony is dyed or heat-treated to deepen and even out its colors. This is a long-accepted trade practice, but dyed stones can fade with harsh chemicals, so it is worth knowing.

The same rule applies to whatever the stone is set in. On affordable jewelry, “18k gold” almost always means 18k gold plating over a base metal, not solid gold. Our own base is 316L stainless steel, a low-nickel alloy rather than a truly nickel-free one, and cubic zirconia, whatever its color, is a lab-made stone, not a diamond and not a natural gem. None of that is bad; it is simply the difference between a piece you enjoy and replace and an heirloom you insure, and you deserve to know which one you are buying.

How to care for each stone

• Peridot. Clean it with warm, soapy water and a soft cloth. Jewelers of America specifically advises against ultrasonic and steam cleaners for peridot, and warns to avoid acids, extreme heat, and sudden temperature changes, since rapid heat can crack the stone. Take peridot rings off before cleaning with household chemicals.
• Spinel. The easy one. At Mohs 8, spinel handles warm soapy water and normal everyday wear without drama.
• Sardonyx. Warm soapy water and a soft cloth again. Keep dyed pieces away from harsh cleaners and prolonged sun so the color stays true, and store banded stones separately so harder gems do not scratch them.

Meaning and symbolism, as history

The lore is a lovely part of these gems, as long as it is read as tradition rather than a property of the stone. Ancient Egyptians called peridot the “gem of the sun” and, by those accounts, set it in gold believing it warded off nightmares; Roman soldiers are said to have worn sardonyx carved with Mars, the god of war, for courage in battle. These are cultural stories, not effects the gem produces.

Frequently asked questions

What is the real birthstone for August?

Peridot is August’s primary modern birthstone, so it is the one on most charts. Sardonyx is the older, traditional stone, and spinel was added as a third official option in 2016. All three are legitimate August birthstones.

Is peridot a good stone for everyday wear?

Yes, with a little care. Peridot is 6.5 to 7 on the Mohs scale, which is durable but sits right at the hardness of everyday dust. It is ideal for earrings and pendants and fine for rings that are not worn around the clock. For a ring you truly never remove, spinel at Mohs 8 is the sturdier August choice.

Is August peridot ever fake or lab-made?

There is no commercial lab-grown peridot, so a stone sold as peridot is either the natural mineral or an imitation. Jewelers of America notes that peridot imitations are typically glass, cubic zirconia, or synthetic corundum, all of which must be disclosed. If a peridot looks too clean and costs very little, ask the seller exactly what it is.

What is the difference between spinel and ruby?

They are different minerals. Spinel is magnesium aluminum oxide, while ruby is aluminum oxide, the mineral corundum. Because large red spinels look like rubies, several famous historical “rubies,” including the Black Prince’s Ruby in the British Imperial State Crown, were later identified as spinel.

How do I clean peridot jewelry?

Use warm, soapy water and a soft cloth, then dry it gently. Do not use ultrasonic or steam cleaners on peridot, and keep it away from acids, high heat, and sudden temperature changes, all of which can damage the stone.

The one rule that covers all three August stones: know what you are buying. A mined peridot, a synthetic spinel, and a dyed sardonyx can all be beautiful, as long as the label is honest about which is which. For more on the stones in the sardonyx family, see our guides to black onyx meaning and symbolism and green agate meaning and symbolism.

If you want to wear August’s stone rather than read about it, our Personalized Cross & Birthstone Necklace is made in a August 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.