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Chalcedony vs Agate vs Onyx: One Material, Three Names

By Digital Towns Crystals ยท Last reviewed October 8, 2026

Short answer: all three are chalcedony, the microcrystalline quartz (SiO2, Mohs about 7) built of fibers too fine to see. The names describe pattern. Plain chalcedony has no bands and is usually one soft color, white, gray or blue. Agate is chalcedony with bands, usually curved and following the walls of the cavity it filled. Onyx is chalcedony whose bands are straight and parallel, classically black and white. One caution: soft banded calcite is also sold as "onyx," and it is not chalcedony at all.

Why are these three names so tangled?

Because one is the family and two are its patterns. Minerals.net explains that chalcedony is not scientifically its own mineral species but quartz in microcrystalline form, and that in the gem trade the word usually describes only white or blue material, to set it apart from banded agate and the other named varieties. So a buyer meets "chalcedony" in two senses: the broad mineralogical one that includes agate and onyx, and the narrow trade one that means plain, unbanded stone.

The second layer of confusion is that agate and onyx overlap. Minerals.net's onyx entry notes that black and white banded onyx can be classified as both agate and onyx, but that many require onyx to have banding in roughly parallel lines, while other agates can have wavy or concentric bands. Sardonyx (red-brown and white straight bands) sits in the same overlap. The third layer is the word onyx itself, which has also been used for solid black chalcedony and for banded carbonate rock. The onyx guide untangles that history; this page is about naming a stone in your hand.

Chalcedony, agate and onyx compared

Feature Chalcedony (trade sense) Agate Onyx (chalcedony)
Pattern None, or soft cloudy zoning Bands, curved or concentric; sometimes flat layers too Straight, parallel bands
Typical colors White, gray, lavender, blue Almost any; natural tones are muted Black and white; sardonyx red-brown and white
Transparency Translucent Translucent Translucent to opaque (dark layers)
Habit in nature Botryoidal crusts, stalactites, nodules Nodules and geode linings Flat-banded zones in nodules
Hardness 7 7 7
Luster Waxy Waxy to glassy when polished Glassy when polished
Common treatment Dye (blue, green) Dye (bright colors) Dye (solid black)
Traditional use Cabochons, carvings, beads Slices, tumbled stones, carvings Cameos and intaglios

Shared properties from the International Gem Society, which lists chalcedony at hardness 7, refractive index 1.540 to 1.553 and a waxy to dull luster. Patterns and uses from Minerals.net's agate and onyx entries.

Which name fits your stone? A five-step check

Work through the steps in order. Each one narrows the name.

Step 1: Is it really chalcedony?

Before arguing over pattern, rule out the impostor. Banded calcite and aragonite, sold as Mexican onyx, onyx marble or limestone onyx, look similar in honey, green and cream bands. Minerals.net lists those names as false names for banded travertine or tufa. Draw a steel blade lightly across an unpolished spot or the base. Chalcedony at Mohs 7 is not marked by steel; calcite at 3 is scratched easily, leaving a white powdery line. If the blade bites, the stone is carbonate "onyx," and none of the following steps apply.

Step 2: Are there bands at all?

Look at the stone in daylight and then with a flashlight held against its back. No bands, just even or gently clouded color: in trade usage it is chalcedony, or one of its single-color variety names. Red to orange translucent stone is carnelian; brownish red is sard; apple green is chrysoprase. Blue chalcedony from different localities carries place names; the IGS mentions Mohave and Mt. Airy blues from California and Nevada and African Blue from Namibia.

Step 3: What shape are the bands?

Curved, concentric or wandering bands that follow the outline of the stone are agate. Geology.com explains why: silica deposits in concentric layers around the walls of a cavity, or in horizontal layers building up from the bottom. Bands that are ruler-straight and parallel across the whole face are onyx (black and white) or sardonyx (red-brown and white). Many nodules have both: curved fortification bands around the rim and a flat, level stack in the middle. A carver who cuts only the flat zone gets onyx; the same nodule sliced whole is an agate.

Step 4: Does the color make sense?

Natural chalcedony colors come from trace impurities. The IGS notes that colors are a result of metallic impurities such as iron, nickel, copper and titanium, and that unstained material is often grayish blue. Saturated, uniform blue, green, purple or pink usually means dye. Solid, jet black "onyx" is the most common treated item of the three: the IGS says natural black and white onyx occurs in thin bands and that what is found in stores is almost always dyed. Minerals.net adds that the banded white and black form is usually natural.

Step 5: Gemologist's check

A Chelsea color filter separates the most common dyes. The IGS states that dyed blue and green chalcedony show pink through the filter. Heat is harder to catch: almost any chalcedony can be turned red by heating in an oven because it contains finely disseminated iron compounds, and the IGS lists that heating as undetectable.

How do the three form?

All three grow from silica dissolved in water at low temperature, so the difference is where and how the silica settles.

Plain chalcedony grows as crusts on open surfaces. Minerals.net lists botryoidal, mammillary, stalactitic and nodular forms, and notes that chalcedony also replaces organic material, which is how petrified wood forms and how coral in Tampa Bay, Florida, was preserved as chalcedony. These bubbly crusts and drip stalactites show what chalcedony does when it has space and nothing forcing it into layers.

Agate grows when a closed cavity, usually a gas bubble in basalt, fills layer by layer from the walls inward. Minerals.net observes that because cavities are irregularly and uniquely shaped, each agate forms its own pattern based on the cavity shape. The chemistry of each layer differs a little, which is why bands differ in color and translucency. The agate banding guide goes deeper into the competing explanations.

Onyx is the special case where gravity wins. When silica gel or fine particles settle in still conditions at the bottom of a cavity, they stack in flat, horizontal layers. Collectors call these water-level or gravity bands, and they record which way was "down" when the nodule formed. Thick, contrasting sets of such layers are rare, which is one reason natural black and white onyx is scarce and dyed material fills the market.

Where does each come from, and what does each cost?

The IGS lists plain chalcedony sources including California, Iceland, India, Mexico, Namibia, Nevada, Oregon and Russia. Agate is mined on every continent, with Brazil, Uruguay, Mexico, Botswana and Argentina dominant in the trade. Minerals.net names India, Brazil and Uruguay as onyx sources.

Price is mostly driven by pattern quality and honesty about treatment:

  • Plain chalcedony is valued for even, natural color. Minerals.net notes that in blue chalcedony, the deeper the color, the more desirable the stone. Some blue material also shows a soft adularescent glow that adds value.
  • Agate is cheap in bulk and expensive at the top, where sharp natural banding and strong color meet. A dyed slab sells for little; a crisp, naturally colored Laguna or Condor slice costs many times more.
  • Onyx with genuine black and white straight bands, thick enough to carve, is the scarcest of the three. Dyed black beads and cabochons are inexpensive commodity items. Sardonyx with even, straight bands sells on the strength of its contrast.

Which should you buy for what?

  • For carving and cameos, onyx and sardonyx are the classic choice. Minerals.net notes onyx has traditionally been one of the most important engraving and cameo stones, usually as a dark base with a white top layer, so a carver can cut a white figure on a dark ground.
  • For slices, geodes and backlit display, choose agate. Bands only reveal their structure when light passes through.
  • For specimens, plain chalcedony is the one with natural forms worth collecting: botryoidal crusts, stalactites and pseudomorphs.
  • For everyday wear, all three are excellent. They share hardness 7 and good toughness, and the IGS says chalcedony requires no special care beyond mild detergent, warm water and a soft brush.

The names have old roots. Chalcedony may come from Chalcedon, an ancient seaport now part of Istanbul, according to the IGS, and agate is named after the Achates River (now the Dirillo) in Sicily. In crystal traditions the three are treated as separate stones, with onyx linked to strength and agate to balance; mineralogically they are one material.

Choosing chalcedony, agate and onyx pieces

Ask three questions of any listing: is it chalcedony (or carbonate), is the color natural, and do the bands match the name? A stone sold as onyx with curved bands is an agate; one sold as agate with no bands is plain chalcedony. Neither is a fraud, but the label tells you how carefully the seller looked.

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Frequently asked questions

Is onyx the same as agate?

Chalcedony onyx is a kind of banded chalcedony, so it overlaps with agate. The difference is band shape: onyx bands are straight and parallel, while agate bands are usually curved or concentric. Black and white onyx can be called either. Mexican onyx is a different material, banded calcite, and is not agate.

Is agate a type of chalcedony?

Yes. Agate is banded chalcedony. In mineralogy, chalcedony is the whole family of microcrystalline quartz, including agate, onyx, carnelian, chrysoprase and jasper. In the gem trade, "chalcedony" alone usually means the plain, unbanded, white to blue material, which is why the names seem to compete.

How can you tell if black onyx is dyed?

Solid, uniformly jet black chalcedony is almost always dyed, because natural black layers are thin and occur between white bands. Look at drill holes and chips with a loupe: dyed stones can show a lighter gray core. Natural onyx usually shows at least faint banding or variation in its black.

Which is harder, chalcedony, agate or onyx?

They are the same hardness, about 7 on the Mohs scale, because they are the same silica. Only carbonate "onyx" is different: banded calcite rates about 3 and is scratched by a steel knife. If your onyx scratches easily, it is calcite rather than chalcedony.

Is blue chalcedony a kind of agate?

Usually not. Most blue chalcedony is unbanded, so it keeps the plain chalcedony name. Blue lace agate is the exception: it is blue chalcedony with fine, wavy bands, which is why it carries the agate name. If the blue is very saturated and uniform, check for dye.

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