Building Materials are a new item in Mr. Mine, (released on the 14th of May 2021) that are available from chests and excavations. They are used to craft the Chest Collector, Metal Detector, Trading post, Manager, Oil Rig and Gem Forge as well as being used to level up those same buildings in the game. The Metal Detector and Manager are no longer available from chests. You can collect 1 Building Material from a regular chest, and 50 from a gold chest.
the chances for getting BM from chests is
|gold chest||regular chest|
|most cases||3% chance for 20 Building Materials
2.5% chance for 50 Building Materials
|special cases||6.5% chance for 20 Building Materials with (under KM 200 and under or Equal to level 3 metal detector)
you have a 3% chance to get 50 Building Materials with (over 200Km or level 4 and above metal detector)