The term onyx was once reserved for a type of agate, which is in turn derived from quartz. This form of onyx is almost exclusively black-and-white and is often used in jewelry. Most of the onyx found in home decor comes from calcite, a mineral that also forms the basis for travertine, limestone and marble. In fact, its resemblance to the latter is the reason why onyx is sometimes known as "onyx marble."
Like marble, onyx has a smooth, somewhat porous surface. It is also similar to marble in that it has a fairly low density. This makes it softer and more pliable than other natural stone tile, particularly granite. It comes in a variety of mostly earth-toned shades and can be finished in a number of ways, though it is typically honed or polished to a high shine.
Onyx may be used in a number of ways. While it is most often seen in kitchens, onyx is also showing up with increasing frequency in bathrooms, as the basis for sinks, showers, floors and even walls. It can also be fashioned into intricate mosaic inlays for fireplace surrounds, table tops and floor medallions.
Clearly, onyx is a versatile stone, but it is also one of striking beauty. Because it was created by nature, no two pieces of onyx are exactly alike, and with hues ranging from pale gold to deep red, onyx tiles can fit into almost any color scheme. The variations in color and pattern can be enhanced through backlighting, which uses onyx's natural translucence to bring a warm, rich glow to almost any room.
The soft and porous nature of onyx makes it susceptible to staining, etching and scratching, even if it is properly sealed. It is especially vulnerable to liquids, which can soak through the surface and cause permanent staining and etching. This can be particularly problematic in kitchens and bathrooms. Its tendency to scratch makes it a less-than-ideal choice for floors. Onyx is also relatively expensive, comparable in price to granite and other natural stone materials.