What it is confused with
Most commercial sardonyx is dyed agate or onyx. Natural bands have softer, less uniform color than dyed ones.
Banded reddish brown and white
Sardonyx birthstone
Sardonyx, the birthstone for August, is banded chalcedony — alternating layers of sard and onyx, banded reddish brown and white, rated 6.5–7 on the Mohs scale. Traditionally, it is linked to steadiness, discipline and durable commitments — attributed, not demonstrated.
- Mohs hardness
- 6.5–7 scratches window glass
- Crystal system
- Trigonal
- Pliny, AD 77
- Book 37.23 as “sardonyx”
- Main sources today
- India also Brazil, Germany
Pliny, Natural History, tr. Bostock and Riley.
Is Sardonyx a birthstone?
Yes. Sardonyx is the traditional birthstone for August.
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Properties
Composition
Banded chalcedony — alternating layers of sard and onyx
Hardness (Mohs)
6.5–7
Crystal system
Trigonal
Color
Banded reddish brown and white
Mined in
India, Brazil, Germany, Czech Republic
Worth knowing
Its parallel banding made it the preferred material for cameos — carvers cut through a pale layer to reveal a dark one beneath.
History
The Romans carved seals and cameos from it; lore says hot wax would not stick to its surface.
Buying
Buying Sardonyx: fakes, value and treatments
What decides the price
An inexpensive stone; sharp, straight contrast between the reddish-brown and white layers is preferred.
Where it is used
The classic cameo stone — a carver cuts through the pale layer so the figure stands out against the dark one.
A less familiar fact
The name combines sard, from the ancient city of Sardis, with onyx.
Practical
Caring for Sardonyx
How to clean it
Warm soapy water and a soft cloth. Avoid prolonged soaking.
How to tell it is real
Banded agate is often dyed to imitate sardonyx. Natural banding is irregular; dyed banding is too even and the color sits in the porous layers.
How hard is Sardonyx?
Sardonyx is layered chalcedony, the stone of Roman cameos, and tough enough for a signet ring; the one thing it dislikes is a long soak.
Sardonyx, at 6.5–7, is harder than 19 of the other 42 stones here and softer than 15, with 8 level with it.
Primary source
What Pliny the Elder wrote about Sardonyx
“The first Roman who wore a sardonyx, according to Demostratus, was the elder Africanus, since whose time this stone has been held in very high esteem at Rome…”
“By sardonyx, as the name itself indicates, was formerly understood a sarda with a white ground beneath it, like the flesh beneath the human finger-nail; both parts of the stone being equally transparent.”
Pliny the Elder, The Natural History, translated by John Bostock and H. T. Riley (London: H. G. Bohn, 1855–57), book 37, chapter 23 — read the original text.
Pliny’s sarda is carnelian; sardonyx, as he describes it, is carnelian layered over white — the banded stone Roman engravers favored for seals. Who Pliny was and why we quote him →
Traditional associations
Sardonyx is traditionally linked to steadiness, discipline and durable commitments. Chakra in crystal practice: Root. Element: Earth.
Questions about Sardonyx
What is Sardonyx made of?
Banded chalcedony — alternating layers of sard and onyx, crystallizing in the trigonal system.
Which month is Sardonyx the birthstone for?
August.
Where does Sardonyx come from?
India, Brazil, Germany, Czech Republic.
Can Sardonyx be worn every day?
Yes. It rates 6.5–7 on the Mohs scale, and the main thing to watch is long soaking.
Sources
Statements of fact on this page were checked against these sources in October 2026. How we choose and check sources: editorial policy.
Cite this page
AstroHot. “Sardonyx Birthstone: The August Stone and Its Meaning.” Accessed October 28, 2026. https://astrohot.com/crystals/gemstones/sardonyx/
For Wikipedia
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