Gem onyx vs. décor “onyx”

In gemology, onyx = straight‑banded chalcedony. In décor, “onyx” often means banded calcite (soft, acid‑sensitive). If it’s a ring stone, it’s almost certainly chalcedony; if it’s a glowing countertop slab, it’s very likely calcite.

Natural & treated black

Jet‑black chalcedony layers do occur but are scarce. Historically, cutters used sugar–acid carbonization or modern dyes to deepen dark layers. Treated onyx is standard in the trade—just label it clearly.

Layered silica gels

Silica‑rich fluids deposit thin gels in cavities and fractures. When conditions stay steady, layers settle parallel like pages—unlike the wavy fortifications of typical agate.

Where the black comes from

Dark layers hold microscopic pigments—carbonaceous matter, iron compounds, or inclusions that absorb light. Treatments deepen this absorption for a true “ink” base.

Why it carves beautifully

Chalcedony’s tight microfibers take crisp detail without brittle chip‑outs, and banding supplies the light/dark two‑tone canvas cameo artists adore.

Palette

• Jet black — natural or treated chalcedony base.
• Snow white cap — cameo‑friendly layer.
• Soft grey — transitional bands for subtle relief.
• Sard brown — warm base in sardonyx variants.

Quality rough shows even, parallel layers with a controllable white thickness over a clean black body.

Pattern words

• Two‑ply — thin white over dark (nicolo potential).
• Multi‑ply — several alternating bands for complex cameos.
• Edge halo — warm translucency at thin edges under backlight.
• Solid black — uniform black chalcedony, no visible bands.

Band anatomy

Look for straight, parallel boundaries. In nicolo material the white may be extremely thin, producing a smoky blue‑grey cameo relief.

Natural vs. dyed

Dyed stones can show color pooling in micro‑cracks/pores and an ultra‑even, jet tone. Natural dark layers may have faint brownish undertones or subtle zoning.

Imitation checks

Glass/obsidian lack banding and may show bubbles; calcite “onyx” scratches easily and fizzes with mild acid. True onyx is quartz‑hard and does not fizz.

Obsidian & black glass

Uniformly black, conchoidal, no bands; glass often has bubbles. Onyx shows parallel layering and a waxier glow.

Jet & black jade

Jet feels light/warm (low SG); black jade is tougher with fibrous/granular texture under magnification. Neither offers a natural white cap for cameos.

“Onyx marble” (calcite)

Decorative banded calcite—soft (Mohs ~3), acid‑reactive, and often translucent in large panels. Beautiful, but a different stone entirely.

Quick checklist

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

Where it shows up

Banded chalcedony suitable for onyx occurs in Brazil, Uruguay, India, Madagascar, and volcanic fields across the USA. Historic cameo rough was long associated with the Mediterranean trade.

What people make

Signet rings, cufflinks, beads, inlay, and of course cameos. Solid‑black cabochons pair cleanly with silver or yellow gold; two‑ply slabs make relief work sing.

Everyday care

Jewelry guidance

• Great for daily wear rings & pendants; quartz hardness holds polish.
• For cameos, a slightly matte background behind glossy relief adds depth.
• Open‑back pendants let thin edges show a gentle halo glow.

On the wheel

• Select rough with the right cap thickness (nicolo = ultra‑thin; bold relief = thicker).
• Orient bands parallel to the face; pre‑polish 600→1200→3k.
• Finish with cerium or alumina on leather/felt for a calm, even sheen.

Edge‑light reveal

Hold a slab to a phone flashlight: the white cap glows, the black base stays dark. It neatly explains cameo contrast.

Feel the difference

Place a small piece of “onyx marble” next to chalcedony onyx. Fingernails tell the tale: calcite scratches, chalcedony doesn’t.