Diagenesis

Diagenesis is the set of processes that convert deposited sediments into a sedimentary rock. These processes include chemical, physical and biological changes of the sediment. Diagenesis takes place at a depth of a few kilometers in the upper crust. For instance deposited mud turns to shale, sand turns to sandstone and gravel turns to Conglomerate as the result of Diagenesis.
The following distinct processes are included in Diagenesis.
1.  Compaction
The pressure due to the overburden causes the compresses the sediments forcing air and water out. This is also known as Consolidation.
2.  Re-crystallization
This refers the formation of new mineral crystals over existing grains. For example, Quartzite sandstone is formed by the development of silica on quartz grains.
3.  Cementation
This is the solidification of loose grains as a result of another mineral acting as a cement. Common cements include Iron oxide, Clay, Silica, and Calcium Carbonate.