Peat is a block most commonly found in swamps. Its primary purpose is a low-tier fuel source for firepits. It burns at Medium heat (~5 notches on the temperature gauge), and burns a little longer than wood. Peat can be collected by either shovel or fist. It is often covered by grass on the surface, but can be distinguished from a regular grass block by its black texture on the block's sides. It can also be found in the middle of large expanses of water.
Prior to build 77, Peat blocks could float, which allowed the player to use as roof material.