In Dwarf Fortress, borax is an unremarkable white stone which can be found in large quantities within rock salt and in small quantities within gypsum.
In real-life, borax is used as powder to either protect wood or is used as a flux while forging iron among many other uses (borax is a great aid for hot-forging two metals, for example it is needed to forge "damascus steel").
Due to an error in the raw parsing code, the game attempts to generate borax as small clusters within rock salt and as large clusters within gypsum. This can be fixed by editing the ENVIRONMENT_SPEC tokens in the raws and reversing the order of the stone names. (Bug:1429)