Gem-quality onyx and “onyx marble”

In gemology, onyx means straight-banded chalcedony (quartz). In architecture and decor, “onyx” often refers to banded calcite — also called “Mexican/green/white onyx.” Calcite onyx is softer (Mohs ~3) and reacts with acid; chalcedony onyx has quartz hardness and does not react. Two very different materials sharing the same name.

Family and related types

• Onyx — parallel black/white bands (cameos value the white “cap” over the black base).
• Sardonyx — parallel brown–red (sard) bands with white.
• Nikolo — a cameo where a very thin light layer over a dark one creates a bluish-gray relief.
• White onyx (trade term) — often banded calcite, not chalcedony.

Layered silica gels

Onyx forms when silica-rich fluids deposit successive gel layers in cavities and fissures. When conditions are stable, layers settle parallel — hence the straight stripes.

Color from chemistry + texture

White/gray layers lack chromophores and strongly scatter light; dark layers contain carbon/iron or very fine inclusions. Natural black bands are rare; many historical artifacts used traditional sugar–acid or dyeing techniques to enhance darkness.

Why it engraves so well

Chalcedony's dense microfibers allow for sharp details without fragile chipping, and the striping provides a natural two-tone "canvas" — perfect for cameos and seals.

Palette

• Black — natural or dyed chalcedony base.
• White — cameo "cap."
• Sardo brown — warm sardonyx base.
• Gray — intermediate layers for subtle relief.

The most valuable raw material has even, straight stripes and a light layer on top, whose thickness is controlled over the dark body.

Pattern terms

• Parallel striped — a feature of onyx/sardonyx.
• Two-layered — light cap + dark base for cameos.
• Multilayered — several alternating light/dark layers for creative freedom.
• Edge halo — warm translucency in thin edges before backlighting.

Photography tip: Use broad diffuse light for "true blackness"; add a slight backlight from below to highlight the white cap without washing out the contrast.

Band structure

Onyx shows straight, parallel boundaries. Edges often glow honey white when light passes through. Sardonyx bases are warm brown, not neutral black.

Black: natural or dyed

Dyed stones may show color accumulations in microcracks/pores and overly uniform, “asphalt” black. Natural dark bands are rarer, sometimes with slight brownish tint or faint zoning.

Cameo characteristics

High-quality cameos show controlled cap thickness — thin for delicate details (Nicolo effect), thicker for bold relief. Look for sharp transitions and clean polishing on the highest points.

Obsidian and glass

Obsidian (volcanic glass) is uniformly black and conchoidal, without bands; glass has round bubbles and lower hardness. Onyx layers — a revealing feature.

Agate and black jade

Jet is light (low SG) and “warm”; black nephrite/jadeite is tougher and often shows fibrous structure when magnified. Neither has onyx’s straight white “cap.”

“Onyx marble” (calcite)

Striped calcite in decor — soft (Mohs ~3), often fizzes in mild acid and is scratched by a knife. Beautiful, but not chalcedony onyx.

Howlite/magnesite (dyed)

When dyed black, black pigment clumps are visible in pores; hardness is much lower (Mohs 3.5–4). Under a loupe — clearly a “chalky” texture.

Brief checklist

• Parallel light/dark bands and quartz hardness?
• Translucency of edges and waxy glassy luster?
• No bubbles, no acid fizz? → Gem onyx (chalcedony).

Where it reveals itself best

Chalcedony onyx and sardonyx are found where silica-rich fluids filled cavities: Brazil, Uruguay, India, Madagascar, and volcanic regions of the USA. Historically, sardonyx cameos are associated with India and the Mediterranean.

What is made from it

Cameos and intaglios (white relief figure on dark background), seal rings, beads, and graceful Art Deco inlays. In decor, “onyx” (often calcite) slabs glow green/white beautifully when backlit.

Daily care

• Clean with lukewarm water and mild soap; soft cloth; dry thoroughly.
• Avoid ultrasound, steam, and aggressive solvents on dyed/impregnated stones.
• Store separately; quartz can scratch softer neighbors and be scratched by corundum/diamond.

Jewelry recommendations

• Perfect for seals, rings, pendants and brooches; graphic black/white easily pairs with silver or yellow gold.
• Cameos with a slightly matte background behind the glossy relief add depth.
• Open backs allow thin edges to shine — a subtle luxury accent in pendants.

Polishing

• Choose raw material with the right cap thickness for your design (thin — for the Nicol effect, thicker — for a vivid relief).
• Orient bands parallel to the cameo plane; polish with cerium or aluminum oxide on leather/felt for a calm shine.
• If the material is dyed, keep low temperature and light pressure so the color does not change in microcracks.

Band revelation

Add an onyx slice to a small backlight: the white cap glows, while the dark base remains… dark. Elegantly explains why cameos "pop."

Texture contrast

Place onyx next to "onyx marble." You can feel the difference: quartz hardness versus buttery softness under the nail.