Sardonyx (chalcedony microcrystalline
quartz) is a blend of sard and onyx, with red
bands of sard and white bands of onyx. Onyx, Sard
and Sardonyx all have a hardness rating of 7. It
is incorrect to name this rock onyx without the
word marble. Onyx marble, Sardonyx or Marble stone
is formed from water containing lime in layered
deposits, and always banded, near springs or as
stalactites or stalagmites in caves. Unlike marble,
Sardonyx or Onyx marble are formed when cold water
carrying dissolved calcite drips into underground
caves. As the water evaporates, a thin film of limestone
is left behind. Over many years, these deposits
build up to form pillars, spikes and other shapes.
Limestone, marble, sardonyx, and travertine are
all in the family of organic rock.