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In geology we cannot speak of onyx in the singular, but of onyx in the plural: in fact, there are divided into two great families according to their composition, making this ornamental stones very different from each other. Silica onyx is recognized and classified as a mineral: it is in fact a variety of chalcedony, the massive, compact and microcrystalline form of quartz. This type of onyx has a phylonian- hydrothermal origin: it is formed from magma which, starting from the magma chamber of a volcanic body, solidifies in underground fractures, forming veins. The magma therefore
does not reach the Earth's surface but remains underground, where the microcrystallization of minerals takes place in an environment in which temperature and pressure gradually decrease. Siliceous onyx has an opaque or semi-opaque appearance, a color varying between red-brown, all shades of gray and black, and a hardness of 6 and 7 on the Mohs scale. The other large family includes calcareous onyxes, real sedimentary rocks from a geological point of view, mainly composed of calcium carbonate. Calcareous onyx is in fact alabaster, a rock that is formed by the precipitation and recrystallization of
calcium carbonate from particularly hard waters: calcareous onyxes have a banded appearance with a wide range of colors, ranging from shades of gray to brown, pink and even yellow-green. This chromatic variability is due to the presence of oxides or fragments of other rocks, which remain within the stratification during the deposition of the alabaster. Calcareous onyx is found mainly in the East (Iran, Turkey, Pakistan) but also in Italy and Macedonia; silica onyx is instead extracted in South America, for example in Mexico.
T HE GENESIS OF ON Y X
THE GEOLOGIST S OPINION
REPORTAGE
Elia Migliorini Geologist