泉水周围的岩石上覆盖着一层由矿物质沉积形成的多孔石灰华。

The rocks around the spring are covered with porous ___tufa formed by mineral deposits.

这座喷泉的底座是用凝灰岩雕刻而成的，看起来很别致。

The base of this fountain was carved from ___tufa, giving it a distinctive look.