An important rock-forming mineral, crystalline silica (SiO₂) occurs either in transparent hexagonal crystals or in crystalline or cryptocrystalline masses. Quartz is the commonest mineral next to feldspar and forms the majority of most sands. It is widely distributed in igneous, sedimentary, and metamorphic rocks.

The interval of geologic time between approximately 2.6 million years ago and the present. It is divided into the Pleistocene and Holocene epochs.