Whether a rock sinks or floats in water depends on its density compared to the density of water. Density is defined as mass per unit volume. If the density of a rock is greater than the density of water, it will sink. If the density of a rock is less than the density of water, it will float.

The density of water is approximately 1 gram per cubic centimeter (g/cm^3) at room temperature. Rocks can have a wide range of densities, depending on their composition and porosity. For example, the density of granite is typically around 2.7 g/cm^3, while the density of pumice can be as low as 0.2 g/cm^3.

Therefore, granite will sink in water because its density is greater than the density of water, while pumice will float in water because its density is less than the density of water.