Sard + onyx, both chalcedony

Chalcedony is microcrystalline quartz (SiO₂) with fibers so fine they look uniform. Sard is the brownish‑red member (colored by iron oxides), while onyx refers to straight‑banded chalcedony, traditionally black‑and‑white. Sardonyx mixes the two: warm red bands with white/black contrasts.

What makes it “onyx” vs. “agate”

Both are banded chalcedony. Onyx shows straight, parallel layers; agate often displays curvy, concentric or fortification patterns. Sardonyx is the onyx style with a sard palette.

Rhythmic silica deposition

Silica‑rich fluids circulate through cracks and cavities. As chemistry and temperature shift, thin chalcedony layers precipitate one after another, stacking like pages in a book.

Iron paints the sard

Tiny amounts of iron oxidize to hematite/goethite, tinting certain layers brownish‑red. Cleaner silica layers stay white or grey; carbon/iron‑manganese films can darken stripes toward black.

Why so straight?

In fractures and seams, the growth front is planar, so layers stack as parallel ribbons instead of the curved walls we see in nodular agates.

Palette

• Sard red‑brown — cinnamon to brick; glows warmly at thin edges.
• Chalk/white — clean boundaries for carving contrast.
• Black onyx — carbon/iron‑rich layers (sometimes enhanced).
• Grey — neutral buffer bands.

Best‑loved sardonyx shows high contrast and even, parallel spacing—nature’s pinstripes.

Pattern words

• Ribbon banding — straight stripes across the slab.
• Layer stack — several thin whites capping a thicker red (perfect cameo canvas).
• Book‑page seams — multiple paper‑thin layers with delicate shading.

Photo tip: Back‑light thin edges to make the sard bands glow; side‑light ~30° to reveal the crispness of the white layers.

Straight vs. curved

In sardonyx, band contacts are parallel and straight. In agate, they’re curved/fortified. That geometry is a fast visual tell.

Natural vs. dyed cues

Dyed zones can look too perfectly uniform; color may pool along surface fractures or drill holes. Natural sard shows soft tonal variation within the red bands.

Micro‑texture

At 10×, chalcedony shows a tight, sugary‑to‑fibrous texture. White bands can look slightly more granular; red bands often show tiny iron‑oxide speckling.

Agate (banded chalcedony)

Often curved, concentric, or fortification bands. Gorgeous—but not the straight‑line look that defines sardonyx.

“Onyx marble” (calcite)

A trade misnomer for banded calcite/aragonite. Much softer (Mohs 3), reacts with acid, and feels warm/soapy compared to chalcedony.

Glass/resin laminates

Layered plastics or glass can imitate stripes but show bubble trails, mold lines, and lack chalcedony’s subtle translucency.

Banded jasper

Opaque silica with earthy tones; lacks the waxy translucency and fine fibrous look of chalcedony in thin edges.

Carnelian & plain sard

Uniform reddish chalcedony with minimal banding. Beautiful, but without the crisp white contrasts that make sardonyx cameo‑friendly.

Quick checklist

• Are bands straight and parallel?
• Do reds show natural tonal shifts, not painted‑on uniformity?
• Is the feel glassy‑waxy (quartz), not soft/soapy (calcite)?

Where it’s found

Quality sardonyx occurs in India (historic cutting centers), Brazil, Uruguay, Madagascar, and parts of the USA. Each region’s chemistry tweaks the warmth of the sard and the brightness of the whites.

Carving heritage

Ancient Greeks and Romans carved sardonyx into cameos, intaglios, and signets, using the white layer for the raised relief and the darker layer for background shadow. Later, Idar‑Oberstein artisans in Germany became famous for refining color contrast—sometimes with clever dye work.

Everyday care

• Clean with lukewarm water + mild soap + soft cloth/brush; rinse & dry.
• Store separately—quartz is tough and can scratch softer neighbors.
• For potentially dyed stones, avoid harsh chemicals, prolonged soaking, steam, and hot ultrasonics.

Jewelry notes

• Excellent for cabs, cameos, signet rings. Bezel settings protect edges.
• For rings, a low dome keeps bands safe from knocks; for pendants, show off the stripe alignment.
• Warm metals (yellow/rose) amplify sard; white metals clean up the monochrome onyx look.

On the wheel

• Orient the slab so the white cap sits where you want relief in a cameo.
• Pre‑polish thoroughly (to 3k–8k) → finish with cerium/oxide on a soft pad.
• Feather edges: slight bevels reduce chipping along band boundaries.

Glow check

Hold a thin edge to a light: natural sard often shows a warm inner glow. Dyed blacks stay opaque and stark—useful for ID.

Ruler test (no math!)

Place a clear ruler atop a slab: do the bands run parallel to the markings? That straightness is classic sardonyx. If the stripes curve away, you’re in agate country.