Sardonyx
Sardonyx is a banded variety of chalcedony — the same cryptocrystalline quartz family as agate, carnelian, and onyx — defined by its specific combination of colors: reddish-brown sard layers alternating with white or black onyx layers. It isn’t a separate mineral species from onyx or carnelian; the distinction is entirely about which bands are present and how they’re arranged. Because it’s a banding pattern rather than a fixed chemistry, sardonyx can vary noticeably in how prominent, straight, or contrasting its layers are, which affects both its visual appeal and, historically, its suitability for the carved seal and cameo work it was long prized for — the contrasting layers let a carver cut a raised design in one color against a background in the other.
The material has an unusually long documented history in engraved gems and signet rings, valued in antiquity precisely because its banding made intaglio and cameo carving visually striking in a way solid-colored stones couldn’t match. India, Brazil, and Uruguay are among the modern sources for gem-quality sardonyx.
Like other chalcedony varieties, sardonyx sits at Mohs 6.5 to 7 — solidly durable for everyday jewelry, resistant to scratching, and well suited to rings as well as the carved pieces it’s traditionally associated with. It requires no special care beyond what any quartz-family stone needs: avoid harsh chemicals, clean with a soft cloth, and it will hold up well over decades of wear. That combination of hardness and stable, unfading color is also why sardonyx cameos and intaglios carved centuries ago still survive in wearable, legible condition today, unlike softer carved materials that degrade or dull with handling.
Sardonyx has the most consistent birthstone record of any stone in this set: it’s listed for August on all four monthly systems tracked on this site — the historical list (with carnelian, moonstone, and topaz), the US 1912 list (with peridot), the UK 2013 list (with peridot again, listed second), and the current US list (with peridot and spinel). In every case it’s part of a pair or trio rather than standing alone, but its presence across every era of list-making, from pre-standardization sources through the 2019 AGTA-extended current list, is genuinely unusual continuity for a birthstone. It doesn’t appear on either zodiac system tracked here.
Gemmological properties
| Mohs hardness | 6.5–7 |
|---|---|
| Crystal system | Trigonal (cryptocrystalline) |
| Chemical formula | SiO2 |
| Refractive index | 1.53–1.54 |
| Color family | Multicolour, Red, Brown |
| Variety of | chalcedony |
| Also known as | — |
| Origins | India, Brazil, Uruguay |
| Organic material | No |
Sardonyx is banded chalcedony combining layers of reddish-brown sard with white or black onyx.
Where Sardonyx appears as a birthstone
By month
Frequently asked questions
- Is sardonyx a different mineral from onyx?
- No — both are varieties of banded chalcedony, distinguished by which colors appear in their layers. Sardonyx specifically combines reddish-brown sard with white or black onyx bands, while onyx alone usually refers to black-and-white or solid black banding. They're the same underlying material, named differently by color pattern.
- What is sard, and is it different from sardonyx?
- Sard is the reddish-brown chalcedony layer itself — essentially a deep-toned relative of carnelian. Sardonyx is the banded stone formed when sard layers alternate with onyx (black or white) layers; sard by itself, without that banding, is a related but separate trade name.
- Why does sardonyx appear on almost every August list?
- It's genuinely consistent — sardonyx is an August stone on all four monthly systems tracked here (historical, US 1912, UK 2013, and current US), though always as part of a pair or trio rather than August's sole stone. It doesn't appear on either zodiac system.