Onyx Identification Sheet

Onyx is a variety of chalcedony — cryptocrystalline quartz intergrown with moganite — distinguished by its characteristic parallel banding in alternating black and white, though monochromatic and brown-banded varieties (sardonyx) also occur. The banding is composed of microcrystalline silica layers deposited sequentially, typically in cavities within volcanic or sedimentary rock. Hardness 6.5–7 and a vitreous to silky luster give it excellent durability for lapidary work; it takes a high polish and has been carved into cameos and intaglios since antiquity. Two common traps for collectors: (1) most commercial "black onyx" has been artificially darkened using ancient sugar-and-acid treatment techniques — natural deep-black onyx is far rarer than the market suggests; (2) materials labeled "onyx" at gem shows frequently include banded calcite from Mexico ("Mexican onyx"), banded alabaster, and banded opal, all of which are much softer than true chalcedony. A simple hardness test sorts them out. The parallel banding that defines true onyx is also what makes it unmistakable when you know what you are looking at: precise, flat, parallel layers — not the irregular banding of agate — formed in an orderly, layer-by-layer process that mirrors the steady accumulation of knowledge. A fitting mineral to have anchoring an AI-assisted knowledge base.