TRAVERTINE

n. type of limestone deposited at the mouth of a spring

Travertine is a sedimentary rock. Travertine is a natural chemical precipitate of carbonate minerals; typically aragonite, but often recrystallized to or primarily calcite; which is deposited from the water of mineral springs (especially hot springs) or streams saturated with calcium carbonate. When pure, travertine is white, but often is brown to yellow due to impurities. When carbon dioxide-rich water percolates through rocks in limestone areas, the water dissolves the limestone and becomes saturated with it. When the water resurfaces later, the sudden drop in pressure and the change in temperature cause the water to release the carbon dioxide gas, much like fizzy drinks. The calcium carbonate then recrystallizes, often over minute underwater plants. The resulting rock is typically quite porous with numerous cavities. When exceptionally porous it is known as calcarious tufa.
See more at Wikipedia.org...

n. travertine, type of limestone deposited at the mouth of a spring

(n.)
A white concretionary form of calcium carbonate, usually hard and semicrystalline. It is deposited from the water of springs or streams holding lime in solution. Extensive deposits exist at Tivoli, near Rome.
  

Crystalline deposits of calcium carbonate precipitated from solution, often found in caves.