Tourmaline, a semiprecious mineral, is a natural source of negative ions*. Within the Biostone filter it increases alkalinity and discourages bacterial growth. It is also a mild source of far infrared radiation (FIR), which assists in the electrolysis (ionization process) that follows filtering.

 

*Tourmaline emits negative charge of any significance only when external heat, pressure or kinetic shock is applied. By itself, sitting as a "lump" of crystal, its electrical emission is tiny. In an ionizer, water flow provides the "kinetic shock" to stimulate a small electric charge. Scientific references for this property of tourmaline can be found on the web.