Porosity

Porosity is the ratio of pores (micro-voids) in the stone, to its total solid volume.
Pores and the capillary structure develop differently in each of the three stone groups.

Igneous Stones :

The formation process of these stones under high pressure and
temperature allows very little open pore space. However, the quartz
grains contract more than half their volume during the cooling process,
and extensive cracking develops across and around the quartz grains.
The grain size of granite and granite-type stones range from small to
large and a particular variety may be composed of two grain sizes,
this increases its porosity. It can be said that granite has more of a
fracture system than a pore system.

Metamorphic Stones :

During the re-crystallization process of limestone to marble the
small grains of calcite are reorganized into a larger crystal structure
and are being repacked into a new texture. It is during this growth
and repacking that micro-spaces (pores) develop between the
grains. Mineralogy and degree of metamorphism causes sizes and
shapes of pores to differ in quartzite and slate.

Sedimentary Stones :

Pores develop as a result of compaction and the cementing process.
A nearly unlimited variety of pore sizes and shapes are characteristic
of sedimentary stones.