Many igneous and sedimentary protolith rocks that are the precursors to metamorphic rocks contain grains in sizes ranging from pebbles to nearly invisible silt particles. As underground pressures and temperatures increase, grains become compacted together, eliminating the pore spaces between the grains and sometimes making it hard for humans to see the grains at all without magnification. On the other hand, some metamorphic rocks still have grains that are visible to the human eye. Slate and phyllite are two types of fine-grained metamorphic rock. Gneiss (pronounced "nice") has medium to coarse grains and may resemble the igneous rock granite. Geologists use grain sizes and colors to help them identify different types of metamorphic rocks in the field.