Onyx
Onyx is a variety of chalcedony — the same cryptocrystalline quartz family that produces agate, carnelian, and jasper — distinguished by its banding: straight, parallel layers running through the stone, classically in black and white, though the name is also used loosely for solid black material. Gemologically, “onyx” describes a banding pattern more than a fixed color, which is part of why the term gets used more freely in the jewelry trade than a strict mineralogist might prefer.
That looseness matters most for the black onyx sold widely today. Naturally-occurring solid black chalcedony exists but isn’t especially common or uniform, and a great deal of commercial “onyx” jewelry is in fact grey or pale chalcedony that has been dyed to a deep, even black — a treatment with a very long history in the trade (documented since antiquity in some form) rather than a modern shortcut. It’s not a deception when disclosed, but a buyer specifically wanting untreated natural material should ask, since visually the two are indistinguishable without lab testing.
Like other chalcedony varieties, onyx sits at Mohs 6.5 to 7 — durable enough for regular wear, resistant to everyday scratching, and a sensible choice for rings as well as pendants and beads. It takes a good polish and holds it well, needing little more than the usual soft-cloth cleaning and avoidance of harsh chemicals.
On the historical 15th–20th-century monthly list, onyx is a secondary July stone, paired with turquoise, in an era before the trade associations settled on ruby alone for that month. Its more distinctive appearance on this site is in the zodiac tables: onyx is the traditional tropical-zodiac stone for Leo. That’s not an arbitrary astrological assignment — it follows directly from the historical monthly table, since Leo begins in late July, the same month onyx and turquoise were listed for on that older table. The modern jewelry-trade zodiac list breaks from this logic entirely, assigning peridot to Leo instead — one instance of a wholesale divergence in which the two zodiac systems name a different stone for every one of the twelve signs, with no sign of agreement anywhere between them. Onyx doesn’t appear on the US 1912, UK 2013, or current US monthly lists, or on the modern-trade zodiac list — its birthstone identity here is specifically historical.
Gemmological properties
| Mohs hardness | 6.5–7 |
|---|---|
| Crystal system | Trigonal (cryptocrystalline) |
| Chemical formula | SiO2 |
| Refractive index | 1.53–1.54 |
| Color family | Black, White |
| Variety of | chalcedony |
| Also known as | — |
| Origins | Brazil, India, Uruguay, Madagascar |
| Organic material | No |
True onyx is banded chalcedony with straight black-and-white (or solid black) bands; much commercial "black onyx" jewelry is dyed chalcedony rather than natural material.
Where Onyx appears as a birthstone
By month
| Month | System | Confidence |
|---|---|---|
| July | Historical (15th–20th century) | high |
By zodiac sign
| Sign | System | Confidence |
|---|---|---|
| Leo | Traditional tropical zodiac | high |
Frequently asked questions
- Is black onyx jewelry usually natural?
- Often not. True onyx is banded chalcedony with straight black-and-white or solid black layering, but solid black chalcedony is uncommon in nature. A large share of commercial 'black onyx' is dyed grey or lightly-colored chalcedony processed to a uniform black — a legitimate, long-established trade practice, but worth knowing if you're buying for the natural-stone story rather than the look.
- How is onyx different from sardonyx?
- Both are banded chalcedony. Onyx traditionally means straight black-and-white (or solid black) banding; sardonyx specifically combines reddish-brown sard with white or black onyx layers. They're the same material family distinguished by which colors appear in the bands.
- Where does onyx appear on birthstone lists?
- It's a secondary July stone (with turquoise) on the older historical list, and it's the traditional tropical-zodiac stone for Leo — a pairing that traces back to Leo's start month of late July on that same historical table.