A soft, porous, chalk-like deposit of calcium carbonate that accumulates around hot springs or cool springs, lighter and more fragile than travertine.
From 'calc-' (calcium) + 'tufa' (from Italian 'tufo,' a porous rock). A geological term combining the calcium reference with the porous rock type.
Calctufa is so light and porous that it's sometimes called 'sponge stone'—ancient Romans used it to insulate buildings, and it's so delicate you can almost carve it with your fingernail!
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