This experimental study determines the minimum boron concentrations required to form tourmaline under hydrothermal conditions. Results show that temperature influences boron thresholds going from 2 wt % at 400 °C to 3.4 wt % B₂O₃ at 600 °C. Thermodynamic models are able to reproduce the same behavior, but the values show a mismatch if not refined to account for the chemistry of the experimental results. Those new constrains are of great help in predicting natural fluid–rock interactions and ore deposit formation.