pumice

Some rocks, like gray volcanic pumice, are laced with air pockets created when gases expanded inside the rock while it was still molten.

有些岩石（像灰色的火山浮石）会布满气窝，这是岩石还在熔融态时，内部气体膨胀而成。

As long ago as 5000 BC the Egyptians were making a tooth powder consisting of powdered ashes of ox hooves, myrrh, powdered and burnt eggshells, and pumice.

早在公元前5000年，埃及人就在制作牙粉，其成分包含牛蹄粉灰、没药、粉状和烧焦了的蛋壳以及浮石。