Answer:

tufa

Answer	Crossword Clue
TUFA	Limestone deposite
TUFA	Porous limestone.
TUFA	Porous rock.

TUFA	Soft, porous rock.
TUFA	Volcanic rock.
TUFA	Volcano rock.
TUFA	Porous rock
TUFA	Geyseral deposit
TUFA	Porous limestone
TUFA	Lime spring deposit
TUFA	Soft, porous rock

Crossword

a puzzle consisting of a grid of squares and blanks into which words crossing vertically and horizontally are written according to clues.