Translucent stones like onyx have a fragile beauty and depth unmatched by other stones. This exotic type of marble literally transmits light, and seems to glow when installed properly. To ensure it keeps these qualities, it requires special handling during the installation process.
While it’s called by another name, onyx is actually a type of marble. Unlike most marbles, however, stones like onyx are so translucent that anything behind them will affect their color. This means that even your setting material has the ability to affect how the final installation looks. For this reason, when installing onyx or very white stones like Alba Chiaro, always use a white thinset mortar. Using gray will darken or dull the appearance of the stone.
Many types of very translucent stone are also very fragile. To help strengthen them, manufacturers may put a fiberglass mesh on the backs of the stones. Special care must be taken to ensure this mesh does not show through the stone after it is installed.
In addition to not wanting the mesh to show through, you need to also take care not to let grooves or marks in the setting material show through the stone. While grooves are typically left in mortar to help bond the stone properly, these grooves will show up as shadows that can be seen through the mortar.
To help prevent this, and to help prevent the mesh from showing through, take the time to smooth out the grooves in the mortar before setting the stone. A perfectly smooth, flat setting surface will produce the best results.
Whether you intend to backlight your onyx, or you just want a unique wall or floor installation, pay close attention to preserving its light transmitting properties. The end results will take on a depth of color and light completely unmatched in other types of marble.