The building stones of ancient Egypt are those relatively soft, plentiful rocks used to construct most temples, pyramids, and mastaba tombs. They were also employed for the interior passages, burialchambers, and outer casings of mud-brick pyramids and mastabas. Similarly, building stones were used in other mud-brick structures of ancient Egypt wherever extra strength was needed, such as bases for wood pillars, and lintels, thresholds, and jambs for doors. Limestone and sandstone were the principal building stones employed by the Egyptians, while anhydrite and gypsum were also used along the Red Sea coast. A total of 128 ancient quarries for building stones are known (89 for limestone, 36 for sandstone, and three for gypsum), but there are probably many others still undiscovered or destroyed by modern quarrying.