“Pisoids are rounded spheroids carbonate grains, composed of concentric laminae which surround organic or inorganic nucleus.
The mineral waters from which they originate are of the sulphurous carbon type, earthy alkaline bicarbonate sulphate. They gush out at a constant temperature of 37.5° from a residual volcanic activity of the nearby Mount Amiata (extinct Pleistocenic volcano) and through an underground path lasting 40 years they mineralize by absorbing carbon dioxide. The minerals dissolved in the waters and coming from the rocks crossed by the effect of CO2 redeposited releasing sulfur, calcium, magnesium and sulphates, thus forming the well-known travertines and these particular limestone formations.
The individual grains are greater than 2 millimeters, but can also exceed 2 centimeters across.