Sardonyx Identification Guide

What Sardonyx Looks Like

Sardonyx is a banded variety of chalcedony combining sard (brownish-red to brown layers) with onyx (white or black layers) in straight, parallel bands. The hallmark is its layering: flat, even, sharply contrasting stripes of reddish-brown and white (sometimes black), unlike the curved or concentric banding of agate. It is translucent to opaque, with a waxy to glassy luster, and the strong color contrast between layers makes it ideal for carving cameos, where the design is cut in the pale layer against a darker background. Colors are warm — chestnut, mahogany, and orange-brown alternating with milky white.

Step-by-Step Field Checklist

1. Look for straight banding. The defining feature is flat, parallel layers (not the curved fortification bands of agate).
2. Identify the colors. Brown-red (sard) alternating with white or black (onyx) layers confirms sardonyx.
3. Check translucency. Backlight thin edges — the sard layers glow translucent reddish-brown.
4. Test hardness. Mohs 6.5–7; it scratches glass and resists a steel knife.
5. Inspect luster and fracture. Waxy to vitreous luster with conchoidal fracture and no cleavage.
6. Watch for dyeing. Overly vivid, perfectly uniform bands or dye concentrated in cracks suggest treated/dyed agate sold as sardonyx.

Key Diagnostic Tests

• Hardness: 6.5–7, scratches glass.
• Banding: straight, parallel layers (diagnostic versus agate).
• Luster: waxy to vitreous; streak white.
• Fracture: conchoidal, no cleavage.
• Acid: no reaction (silica).
• Specific gravity: ~2.6.

Common Look-Alikes and How to Tell Them Apart

• Agate: also banded chalcedony, but agate bands are curved, concentric, or fortification-shaped; sardonyx bands are straight and parallel.
• Onyx (plain): black-and-white straight banding without the brown-red sard layers; sardonyx specifically includes sard.
• Carnelian/sard: uniformly colored without banding; sardonyx is the layered combination.
• Dyed banded agate: commonly sold as sardonyx; look for unnatural, oversaturated color and dye following fractures, plus suspiciously perfect uniformity.
• Banded jasper: opaque and does not transmit light, whereas sardonyx layers are translucent.

Where Sardonyx Is Found

Sardonyx forms in volcanic vesicles and seams from layered deposition of silica solutions. Major sources include India, Brazil, Uruguay, Germany (Idar-Oberstein), and the United States. It has been a prized carving and seal stone since ancient Egypt, Greece, and Rome, where its contrasting layers were used to cut some of history's finest cameos.