The materials commercially known as Onyx are scientifically known as alabasters (or calcium alabasters) formed by deposits of calcium carbonate in water rich in bicarbonate, generally in cavities in solids of limestone stone.
The term onyx (which came from the Greek term which means “nail”) refers to two different types of rocks: the first is calcium onyx, object of the current article, and the second is a crypto-crystalline variety of quartz.
Both technical as well as aesthetic characteristics vary as a function of the direction of cutting: parallel to the level of stratification (transversal cut), this way a circular design is seen over a layer, or perpendicular (cut following the direction of the veins), in this case the sedimentary stratification is highlighted. Currently the transversal cut prevails, because with this cut the characteristic of translucency is highlighted.
A characteristic of the onyx is the presence of strips, owing to the fact that the levels of carbonate can have different colours, according to the concentration of the minerals and the impurities contained. The most common mineral of “marble onyx” is calcite, followed by aragonite; other minerals frequently associated are clay and quartz. The colour is white or varied (beige, orange, brown, pink, red, green, sky blue, and yellow) due to impurities of the different chemical compositions. The colour of onyx is rarely uniform, thus in the same deposit there can be clear coloured materials alternating with dark or even brown or opaque white colour (variety least appreciated). The veins or brownish red spots are very frequent owing to the presence of iron oxide.
The onyx can be processed to thick cm slabs which, given the easy breakage, are often reinforced with artificial supports and with impregnations of epoxy resin. The resistance of the onyx to atmospheric agents is poor, therefore its use is more adequate for interiors without contact with humidity (kitchens and bathrooms) because even the relatively weak acids can destroy the material; currently the most extended use is in the form of a slab of 0.8 to 1.5 cm thickness and retro illumination (in this case the slab can be polished on both sides).
The most frequent use is in the elaboration of different objects: already five thousand years before Christ vases were made of it in Egypt. Tables, elegant counters, various decorative elements, chimneys, retro illuminated wall claddings are some other applications, apart from use in sculptures.