The density of mercury is about 13.534 g/cm^3 at room temperature. The density of granite lies between 2.65 and 2.75 g/cm^3. So it only takes between 2.65/13.534 ≈ .1958 and 2.75/13.534 ≈ .2032 cm^3 of mercury to hold up 1 cm^3 of granite.
Between 19.6% and 20.3% of the granite will be submerged in the mercury, depending on the density of the granite. (That's about 1/5 of the granite.)
Between 19.6% and 20.3% of the granite will be submerged in the mercury, depending on the density of the granite. (That's about 1/5 of the granite.)