These stones are of the same composition as common marbles, but were formed by chemical deposits instead of in sedimentary beds, crystallized by the action of heat. "They owe their banded structure and variegated colors to the intermittent character of the deposition and the presence or absence of various impurities, mainly metallic oxides. The term onyx as commonly applied is a misnomer, and has been given merely because in their banded appearance they somewhat resemble the true onyx, which is a variety of agate."

Owing to their translucency, delicacy and variety of colors, and the readiness with which they can be worked and polished, the onyx marbles are considered the handsomest of all building stones, and they also bring the highest price, the cost per square foot for slabs 1 inch thick varying from $2.50 to $6. Their use is confined to interior decoration, such as wainscoting, mantels, lavatories, and for small columns, table tops, etc. Most of the onyx marble used in the United States is imported from Mexico, although considerable onyx is quarried at San Luis Obispo, California, and quarries of very beautiful stone have recently been opened near Prescott, Arizona. The Mexican onyx presents a great variety of colors, creamy white, amber yellow and light green, generally more or less streaked or blotched with green or red. Some of the light stones have a beautiful translucent clouded effect. When cut across the grain the stone often presents a beautifully banded structure like the grain of wood. Cutting the stone across the grain, however, greatly weakens its strength, so that it is necessary to back it with slabs of some stronger marble.

The San Luis Obispo stone is nearly white, finely banded, translucent, and takes a beautiful surface and polish.

The Arizona stone presents a greater variety of coloring, from milky white to red, green, old gold and brown, intermingled in every possible way. But a comparatively small amount of this stone is as yet on the market, but further developments will probably result in the production of a great quantity of the stone.