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.
http://www.litosonline.com/en/articles/en/530/onyx-materials-always-fashion
