Digital TownsCrystalsShop crystals
  1. Crystals & Minerals
  2. Onyx

Onyx: One Name, Three Different Stones

By Digital Towns Crystals ยท Last reviewed October 8, 2026

Short answer: "onyx" is used for three different materials. Gemologically it is chalcedony (SiO2, Mohs 7) with straight, parallel bands, black and white in classic onyx and red-brown and white in sardonyx. Solid black "onyx" beads are almost always chalcedony dyed black. And the soft, translucent, honey-and-green banded stone sold as "Mexican onyx" or "onyx marble" is not chalcedony at all: it is banded calcite or aragonite, Mohs 3 to 4, that scratches with a knife.

Which "onyx" is which?

Start with the table, because the name alone tells you almost nothing about what is in your hand.

Chalcedony onyx and sardonyx Dyed "black onyx" Carbonate "onyx" (Mexican onyx, onyx marble)
What it is Banded chalcedony, microcrystalline quartz Chalcedony with black dye Banded calcite or aragonite
Formula SiO2 SiO2 CaCO3
Mohs hardness 7 7 3 (calcite) to 3.5 to 4 (aragonite)
Specific gravity about 2.6 about 2.6 2.7 (calcite) to 2.9 to 3.0 (aragonite)
Look Opaque to translucent, sharp straight bands Uniform jet black, high polish Translucent, glows when backlit; cream, honey, green, brown bands
Acid No reaction No reaction Fizzes
Typical use Cameos, beads, cabochons, palm stones Beads, rings, cabochons Carvings, bowls, spheres, lamps, tile

Hardness and gravity figures are from Minerals.net on onyx, Geology.com on calcite and Minerals.net on aragonite.

What is true onyx, mineralogically?

True onyx is a variety of chalcedony, the compact, fibrous form of quartz. Minerals.net defines its most accepted gemstone meaning as solid black chalcedony, or a banded or layered black and white chalcedony, and gives a hardness of 7, a refractive index of 1.54 to 1.55 and a specific gravity of 2.63 to 2.65. The International Gem Society describes onyx simply as a chalcedony with straight bands of colors, and sardonyx as onyx with white and red layers (International Gem Society).

The line between onyx and agate is the shape of the banding. Many authorities, as Minerals.net puts it, require onyx to have banding in roughly parallel lines, whereas other agates can have wavy or concentric bands. A black and white piece with straight layers can fairly be called both onyx and agate.

Straight layering is what made onyx the classic cameo stone. A carver cuts through a white top layer into a dark base, leaving a white figure standing on a black or red-brown ground. Minerals.net notes that onyx was traditionally one of the most important engraving and cameo gemstones, usually with a black base and a white upper layer. The same layered material is what you want in a sardonyx palm stone today: crisp, flat bands running right across the piece.

Where chalcedony onyx comes from

Minerals.net lists India, Brazil and Uruguay as sources of onyx. Much of the banded material on the market comes from the same Brazilian and Uruguayan volcanic agate deposits that supply ordinary agate, cut so the bands lie flat.

Is black onyx natural or dyed?

Almost always dyed. Natural chalcedony with black layers does exist, but it occurs in thin bands, and the International Gem Society says that what you find in stores is almost always dyed. Minerals.net makes the same distinction: black onyx is often dyed, while the banded white and black form is usually natural.

Dyeing works because chalcedony is slightly porous. The color penetrates the fibrous structure and is permanent, so dyed black onyx is a stable, durable gem with the full hardness of 7. That makes it a fair product at a fair price, as long as it is described honestly. In the United States, the FTC Guides for the Jewelry Industry treat marking a product with an incorrect varietal name as unfair or deceptive, and they require treatments that significantly affect value to be disclosed.

Signs of dye in black onyx:

  1. A perfectly uniform jet black with no hint of banding, even under a loupe at a drilled hole.
  2. Identical color across dozens of beads on a strand.
  3. Very low prices for large, even pieces.

None of these is a defect. They simply tell you what you are buying.

What is Mexican onyx, really?

Banded calcite or aragonite. It is a carbonate stone that formed in layers from calcium-rich water, mostly around springs and in caves, and the trade borrowed the word "onyx" because of the bands. Minerals.net lists the names Limestone Onyx, Mexican Onyx and Onyx Marble as false names for travertine or tufa that shows color banding from layer deposition. Its calcite page defines onyx marble as travertine or tufa in the mineral form of aragonite or calcite that exhibits color banding (Minerals.net on calcite).

The aragonite route is the hot-spring one. Minerals.net explains that aragonite often forms as a deposit from hot, mineral-rich springs: the water releases calcium as it emerges and builds mounds and thick crusts, and when these are banded they may be carved and given trade names such as "Onyx Marble" and "California Onyx". Calcite onyx forms the same way at lower temperatures and as cave flowstone. Geology.com illustrates calcite "in the form of translucent onyx from Tecali, Mexico", a classic Puebla carving locality.

Because it is calcium carbonate, this material behaves nothing like quartz:

  • It is soft. Calcite rates 3 on the Mohs scale and aragonite 3.5 to 4, so a steel knife or a quartz grain scratches it.
  • It reacts with acid. Calcite is effervescent in hydrochloric acid and most other acids, which also means vinegar, lemon juice and many bathroom cleaners will etch a polished surface.
  • It glows when backlit. The translucent cream, honey and green layers are why it is carved into lamps and candleholders.

Calling it "Mexican onyx" is an old, widespread trade habit, not a fraud in itself, but the name hides the fact that it needs gentler care than quartz. Read the label as "banded carbonate stone from Mexico", not as a variety of onyx.

Blue and green "onyx"

Colored carbonate onyx follows the same rule. A pale blue banded carving from Mexico labeled "blue onyx" is the same banded calcite or aragonite family in a blue tone, and green "onyx" tiles and bowls are usually the same carbonate in green. Minerals.net adds that "green onyx" can also mean chalcedony dyed deep green, so a green bead may be quartz while a green bowl is calcite. Hardness and acid settle it.

How to tell chalcedony onyx from calcite onyx at home

  1. Backlight it. Hold a flashlight behind a thin edge. Carbonate onyx glows warmly through its bands; chalcedony onyx stays opaque or shows only faint translucency at the edges.
  2. Scratch test on an unseen spot. A steel nail marks calcite and aragonite easily. It will not scratch chalcedony, which rates 7.
  3. Feel the temperature and luster. Polished calcite has a softer, slightly waxy glow; chalcedony takes a harder, glassier shine.
  4. Look at the bands. Chalcedony onyx bands are sharp and flat. Carbonate onyx bands are often wavy, fibrous and translucent, like layered honey.
  5. Acid, last. A drop of dilute acid on a hidden spot fizzes on calcite, but this test is destructive and rarely needed.

How do you care for onyx?

Chalcedony onyx and sardonyx: wash with warm water and mild soap. At hardness 7 they wear well in rings.

Carbonate onyx: dust with a soft dry cloth; wipe spills at once; never use vinegar, citrus or acidic cleaners; keep it away from harder stones that will scratch it; do not put carvings in a dishwasher. Large carvings are heavy for their size and chip at thin edges, so lift them from the base.

What is onyx traditionally associated with?

In crystal traditions, black onyx is associated with protection, self-control and steady focus, and sardonyx with discipline. These are traditions and beliefs, not measurable properties.

Shopping the onyx we stock

Our onyx stock covers both families, which is exactly why the label needs reading. Price follows the material: carbonate onyx is cheap per pound and priced for carving work and size, while chalcedony onyx and sardonyx are priced for clean, straight banding and polish.

  • Onyx: small polished pieces from Mexico, which places them in the banded calcite family, not chalcedony. A good, inexpensive way to see carbonate banding up close.
  • Blue Onyx: a pale blue banded carbonate from Mexico, calcite or aragonite rather than chalcedony; treat it as soft and keep it away from acids.
  • Carved Onyx Aztec Mayan Tiki Sculpture, 4 lbs, 7 inches, Mexican Onyx: a 7 inch hand carving in cream, green and honey banding, the classic use for this soft, translucent calcite stone.
  • Sardonyx palm stone: the chalcedony side of the family; look for flat red-brown and white bands across the face.
  • Sardonyx: small pieces of the same banded chalcedony, hard enough for everyday handling.

When you buy, ask two questions: is this chalcedony or carbonate, and if it is solid black, is it dyed? A seller who can answer both is worth buying from.

Digital Towns Market

Onyx and sardonyx in stock

See all crystals and minerals

Frequently asked questions

Is Mexican onyx real onyx?

No. Mexican onyx is banded calcite or aragonite, a calcium carbonate stone that formed in layers from spring or cave water. True onyx is banded chalcedony, a form of quartz. Mexican onyx is far softer, around Mohs 3 to 4, and reacts with acid.

Is black onyx a natural stone?

The material is natural chalcedony, but the solid black color is almost always produced by dyeing. Natural black chalcedony occurs only in thin bands. Dyed black onyx is stable and durable and is a legitimate product when it is sold as dyed.

What is the difference between onyx and sardonyx?

Both are chalcedony with straight, parallel bands. Classic onyx pairs black with white layers. Sardonyx pairs red to brown layers, from sard, with white or sometimes black ones. Hardness, care and durability are the same.

Can onyx get wet?

Chalcedony onyx and sardonyx can be washed in warm soapy water. Carbonate "Mexican onyx" tolerates plain water briefly, but acidic liquids such as vinegar, citrus juice or wine will etch and dull its polish, so dry it promptly and avoid cleaners.

Is onyx the same as agate?

Banded black and white onyx is technically a kind of agate. The difference is the banding: onyx has flat, parallel layers, while most agates have curved or concentric bands. Carbonate onyx is not agate at all.

Sources