Black Onyx Oval Cabochon · 18×9 mm

18×9 mm oval black onyx cabochon with a high-reflectance mirror finish. Opaque black chalcedony, flat back. Elongated format, ideal for statement rings and central pendants with a 925 silver frame. Stock acquired from a wholesale lapidary market.

Piece Details

This specific piece

The 18×9 mm format —a 2:1 length/width ratio— is one of the most commonly used in artisan jewelry because it offers sufficient surface for a visual statement without exceeding finger width in a ring. The deep black of onyx absorbs almost all incident light, producing a surface that appears as an absence of color rather than a color itself. This optical behavior contrasts effectively with polished metals (bright silver, oxidized silver, copper) and colored gemstones.

How the black color of onyx is formed

Chalcedony is silicon dioxide (SiO₂) with a cryptocrystalline structure: its crystals are so small that they are not individually visible under magnification. Natural black onyx owes its color to inclusions of carbon or organic matter uniformly distributed in the crystal lattice during the formation of silica nodules. However, in the lapidary market, the vast majority of commercial black onyx is gray or white chalcedony that has been impregnated with a sugar solution and carbonized with sulfuric acid —a technique documented since the 18th century— to produce an intense and uniform black. The result is mineralogically identical to natural black onyx, and the process does not affect the durability or workability of the piece.

For what setting does it work?

• Bezel setting (925 silver or other precious metals) — The closed bezel highlights the oval profile and frames the.
• Statement rings / Central pendants in silver — The 18×9 mm format is proportional for a central pendant.
• Wire wrap — Black onyx accepts wire wrapping well; the.

Onyx in jewelry history

Black onyx is one of the gems with the most extensive documented use: it appears in Mesopotamian cylinder seals from the third millennium BCE and in Greco-Roman cameos carved from alternating strata of black and white chalcedony (the white stratum is carved in relief against the black background). In 19th-century Victorian jewelry, black onyx was widely used in mourning jewelry, often combined with human hair or Whitby jet. In contemporary goldsmithing, it is a reference for chromatic sobriety and maximum contrast with silver and gold.

In crystal traditions, black onyx is associated with protection and grounding. It is considered a stone of stability and stress management, symbolically linked to the earth and the consolidation of boundaries.

The symbolic properties attributed to minerals belong to cultural and historical traditions. They are shared for educational purposes, not as medical advice.

How to recognize genuine black onyx

Black onyx —whether natural or carbonization-dyed— is chalcedony and shares its physical properties: slightly cool to the touch, it keeps glass cool longer than glass or resin. Under magnification, the surface shows the same uniform black without variations in depth. Imitations in black glass are lighter (density ~2.5 vs ~2.6 g/cm³ for chalcedony) and show microbubbles in cross-section. Black plastic is easily scratched with a metal point, which does not happen with onyx.

Frequently Asked Questions

Is the black color natural or treated?

Most commercial black onyx, including this stock, is chalcedony with carbonization dye —a standard lapidary process documented since the 18th century.

What are its exact dimensions?

18×9 mm on the major and minor axis of the oval. Tolerance ±0.5 mm as it is lapidary stock without individual micrometric calibration.

Does it withstand daily wear in a ring?

With Mohs hardness 6.5-7, onyx withstands daily wear well with normal caution. We recommend avoiding direct impacts on the edge; a bezel setting adds extra protection.