Ornamental onyx described by the USGS as “onyx marble” is a crystalline form of calcium carbonate deposited from aqueous solutions. It can display layering and translucency. A trade name should be accompanied by material identification.
The Jewel Cave example illustrates calcite precipitation from water and crystalline layering. It is a general process reference, not documentation of the Iranian quarries listed here.
The eight areas in this guide were identified by Reza Molai. Each block’s origin should be linked to a quarry and batch, rather than inferred from colour alone.