Black onyx

Black onyx

Black Onyx • straight‑banded chalcedony (microcrystalline SiO₂) Palette: black base with white/grey cap (cameo grade) • also solid black cabs Mohs: ~6.5–7 • SG: ~2.58–2.64 • Luster: waxy‑vitreous • Cleavage: none • Fracture: conchoidal Note: natural jet‑black layers are uncommon; much market material is dyed or sugar‑treated chalcedony

Black Onyx — Minimalism with a Quiet Glow

Black onyx is chalcedony at its most graphic: calm, parallel layers that read like ink on paper. Jewelers prize two looks—solid black cabochons with a soft glassy shine, and banded black‑and‑white slabs where a pale “cap” over a dark base becomes a ready‑made canvas for cameos and signets. Hold a thin edge to light and you’ll see why designers love it: the white layer glows, the black stays inky, and the contrast does the storytelling.

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What it is
Chalcedony laid down in straight, parallel bands. “Black onyx” may be naturally dark or traditionally treated to achieve deep black—disclosure is part of good labeling
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Why it captivates
Tuxedo‑clean contrast, crisp carving potential, and a mellow polish that flatters both antique and modern designs
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Care snapshot
Quartz‑hard and stable; mild soap + water; avoid ultrasonics/steam & harsh chemicals on dyed stones

Identity & Terms 🔎

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.

Side terminology: Onyx = black/white bands • Sardonyx = brown/white bands • Nicolo = ultra‑thin white cap over dark for a smoky cameo effect.

How Straight Bands Happen 🧭

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.

Think ruled paper for lapidaries: straight lines that let you write in stone.

Palette & Pattern Vocabulary 🎨

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.
Photo tip: Use broad diffused light for honest blacks; add a tiny backlight below the edge so the white cap glows without washing contrast.

Physical & Optical Details 🧪

Property Typical Range / Note
Composition Micro/cryptocrystalline SiO₂ (chalcedony); dark layers from natural pigments or treatments
Crystal system Trigonal (quartz) — crystals too fine to see; aggregate texture
Hardness (Mohs) ~6.5–7 (daily‑wear friendly)
Specific gravity ~2.58–2.64
Refractive index (spot) ~1.535–1.539 (chalcedony typical)
Luster / Transparency Waxy‑vitreous; usually opaque look with translucent edges when thin
Cleavage / Fracture No cleavage; conchoidal fracture; takes a fine even polish
Treatments Dyeing and traditional sugar–acid carbonization common for black; occasional polymer impregnation for porous slabs
Plain‑English optics: the white cap scatters light and reads bright; the black layer absorbs and stays inky—carving simply reveals whichever layer you want to see.

Under the Loupe 🔬

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.

Look‑Alikes & Mix‑ups 🕵️

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).

Localities & Uses 📍

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.

Care & Lapidary Notes 🧼💎

Everyday care

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Mild soap + lukewarm water, soft cloth, dry well. Store separately to avoid scuffs.

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Avoid ultrasonics, steam, bleach, acetone on dyed stones; avoid prolonged heat on assembled cameos.

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.
Display tip: Stage a raw banded slice beside a finished signet—instant “from rock to ring” moment.

Hands‑On Demos 🔍

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.

Black onyx is the little black dress of gemology—effortless, versatile, and surprisingly photogenic.

Questions ❓

Is most black onyx dyed?
Much of the solid jet‑black material is dyed or sugar‑treated chalcedony. That’s long‑standing and acceptable with clear disclosure.

Does dye fade?
Quality treatments are generally stable in normal wear. Avoid harsh chemicals/solvents and prolonged high heat.

Is “white onyx” in décor the same stone?
Usually not—it’s banded calcite used for panels and vases. Gorgeous, but softer and acid‑sensitive compared to chalcedony onyx.

Can I wear it every day?
Yes. With quartz hardness and a forgiving polish, black onyx is a practical choice for daily jewelry.

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