Gem onyx vs. “onyx marble”

In gemology, onyx means straight‑banded chalcedony (quartz). In architecture and décor, “onyx” often means banded calcite—also called “Mexican/green/white onyx.” Calcite onyx is softer (Mohs ~3) and reacts to acid; chalcedony onyx is quartz‑hard and does not. Two very different materials sharing one name.

Family & friendly cousins

• Onyx — parallel black/white bands (white cap over black base prized for cameos).
• Sardonyx — parallel brown‑to‑red (sard) with white.
• Nicolo — cameo cut where an ultra‑thin pale layer over dark creates a soft blue‑grey relief.
• White onyx (trade) — often banded calcite, not chalcedony.

Layered silica gels

Onyx grows as silica‑rich fluids deposit successive gel layers in cavities and fractures. When conditions stay steady, layers settle parallel like pages—hence the straight banding.

Color from chemistry + texture

White/grey layers are low in chromophores and scatter light strongly; darker layers contain carbon/iron or very fine inclusions. Natural black layers are rare; many historic pieces used traditional sugar–acid or dye treatments to deepen the dark base.

Why it carves so well

Chalcedony’s tight microfibers take crisp detail without brittle chip‑outs, and banding provides a natural two‑tone canvas—perfect for cameos and signets.

Palette

• Black — natural or dyed chalcedony base.
• White — the cameo “cap.”
• Sard brown — warm base for sardonyx.
• Grey — intermediate layers for subtle relief.

The most collectible rough shows even, straight bands with a top light layer of controlled thickness over a dark body.

Pattern words

• Parallel banded — defining trait of onyx/sardonyx.
• Two‑layer — light cap + dark base for cameos.
• Multi‑ply — several alternating light/dark layers for design freedom.
• Edge halo — warm translucency at thin edges under backlight.

Photo tip: Use broad diffused light for honest blacks; add a faint backlight at the lower edge to reveal the white cap without washing contrast.

Band anatomy

Onyx shows straight, parallel boundaries. Edges often glow honey‑white in transmitted light. Sardonyx bases look warm brown instead of neutral black.

Natural vs. dyed black

Dyed stones may show color pooling in micro‑cracks/pores and an overly even, jet tone. Natural dark layers are rarer, sometimes with slight brownish undertones or faint zoning.

Cameo clues

Quality cameos reveal a controlled cap thickness—thin for delicate features (nicolo effect), thicker for bold relief. Look for crisp transitions and clean polish on high points.

Obsidian & glass

Obsidian (volcanic glass) is uniformly black and conchoidal, no banding; glass shows rounded bubbles and lower hardness. Onyx’s layered structure is the giveaway.

Jet & black jade

Jet feels light (low SG) and warm; black jade (nephrite/jadeite) is tougher and often shows a fibrous structure under magnification. Neither shows onyx’s straight white cap.

“Onyx marble” (calcite)

Banded calcite used in décor—soft (Mohs ~3), often effervesces in mild acid, and can be scratched by a knife. It’s beautiful—but not chalcedony onyx.

Howlite/magnesite (dyed)

When dyed black, pores can show inky concentrations; hardness is much lower (Mohs 3.5–4). Under a loupe, the chalky texture is obvious.

Quick checklist

• Parallel light/dark bands with quartz hardness?
• Edge translucency and waxy‑glass luster?
• No bubbles, no acid fizz? → Gem onyx (chalcedony).

Where it shines

Chalcedony onyx and sardonyx occur wherever silica‑rich fluids filled cavities: Brazil, Uruguay, India, Madagascar, and volcanic provinces in the USA. Historic sardonyx for cameos was long associated with India and the Mediterranean.

What people make

Cameos & intaglios (white relief on dark ground), signet rings, beads, and sleek Art Deco inlays. In décor, “onyx” slabs in green/white are usually calcite and glow beautifully when backlit.

Everyday care

• Clean with lukewarm water + mild soap; soft cloth; dry well.
• Avoid ultrasonics, steam, and harsh solvents on dyed/impregnated stones.
• Store separately; quartz can scuff softer neighbors and be scuffed by corundum/diamond.

Jewelry guidance

• Excellent for signets, bands, pendants, and cufflinks; the graphic black/white pairs effortlessly with silver or yellow gold.
• For cameos, a slightly matte background behind glossy relief adds depth.
• Open backs let thin edges glow—a subtle premium touch on pendants.

On the wheel

• Select rough with the right cap thickness for your design (thin for nicolo, thicker for bold relief).
• Orient bands parallel to the face for cameos; polish with cerium or alumina on leather/felt for a mellow sheen.
• If material is dyed, keep heat low and pressure light to avoid color shifts along micro‑cracks.

Band reveal

Hold an onyx slab to a small backlight: the white cap glows, the dark base stays… dark. It elegantly explains why cameos pop.

Texture contrast

Place onyx next to a piece of “onyx marble.” Visitors can feel the difference: quartz‑hard vs. buttery‑soft under the fingernail.