Water from the ground, lakes, or oceans seeps into the pores of organic tissue and forms a crystal cast with deposited minerals. Crystals begin to form in the porous cell walls. This process continues on the inner surface of the walls until the central cavity of the cell, the lumen, is completely filled. The cell walls themselves remain intact surrounding the crystals. Permineralization is different from petrification in that the organic material is only filled with minerals and not completely replaced. Permineralization can occur in several ways: