Gerstley Borate is a calcium borate ore containing the mineral Ulexite (3CaO Na2O 5B2O3 16H2O), along with accessory minerals such as Bentonite, Colemanite, and Probertite.   It supplies B2O3, CaO, other fluxes, SiO2, and Al2O3.  It is a source of Boron which is both a flux and viscosity agent.