floatstone

from The Century Dictionary.

• noun A spongiform quartz, a mineral of a spongy texture, of a whitish-gray color, often with a tinge of yellow, so light as to float in water. It frequently contains a nucleus of common flint.
• noun In bricklaying, a stone used to rub curved work smooth and remove the ax-marks, as in the heads and backs of niches. Its form is made the reverse of that of the surface on which it is to be used.