Borax crystals. Borax is a white, monoclinic crystalline mineral composed of hydrated sodium borate. The crystals are normally quite small, and aggregate into the sort of earthy mass seen in this sample. It is soft and very light. Borax forms during solar evaporation of saline water bodies, such as lakes and desert salt pans. It is the main source of boric acid, which is used as a cleaning product.