Onyx Marble (also commonly called Onyx)

Hardness: 3 on Mohs scale; Color: Ranges widely, often banded with shades of white, cream, yellow, brown, green, and sometimes even blue or red, due to mineral impurities; Luster: Waxy to vitreous (glassy); Crystal Structure: Cryptocrystalline to fine-grained crystalline aggregates of calcite; sometimes displays fibrous growth; Cleavage: Perfect rhombohedral cleavage (though rarely seen in polished specimens); Specific Gravity: 2.7-2.9

Formed from the precipitation of calcium carbonate from mineral-rich spring waters, often in caves (as stalactites and stalagmites) or surface springs. Over time, these calcium carbonate deposits build up in layers, creating the characteristic banding. The geological age varies significantly, as it continues to form today in some locations.

Predominantly used for decorative purposes, including countertops, tiles, sculptures, vases, and ornamental objects. Its translucent property makes it popular for lighting features. It is also used in jewelry as cabochons or beads, though less common than other gemstones due to its relative softness.

Despite its common name, 'onyx marble' is not true onyx (which is a form of chalcedony, a cryptocrystalline quartz). It is a form of banded travertine. The term 'onyx' in this context refers to its banded appearance. Its translucent nature allows light to pass through it, creating a beautiful glow, especially when backlit. The distinct banding is a result of varying impurities (like iron oxides for reds/yellows, chlorite for greens, or manganese oxides for browns/blacks) or changes in water conditions during its formation.

In the field, 'onyx marble' can be identified by its distinctive banding and its reaction to acid (it will fizz, indicating the presence of calcite). Its relative softness (can be scratched with a knife) also distinguishes it from harder true onyx. Common locations include Mexico, Pakistan, Iran, and various parts of the USA (e.g., Arizona, California, Utah). For collectors, polished spheres, carvings, and slabs showcasing its beautiful banding and translucence are highly sought after. Always perform an acid test (dilute HCl) or a scratch test to differentiate it from true onyx or other banded stones.