Recent production of long carbyne chains, concurrent with advances in the synthesis of pure boron fullerenes and atom-thin layers, motivates an exploration of possible one-dimensional boron. By means of first principles calculations, we find two isomers, two-atom wide ribbon and single atom chain, linked by a tension-driven (negative-pressure) transformation. We a) explore the stability and unusual properties of both phases, demonstrating mechanical stiffness on par with the highest-performing known nanomaterials, and a phase transition between stable 1D metal and an antiferromagnetic semiconductor, with the phase boundary effectively forming a stretchable 1D Schottky junction. In addition, the two-phase system can serve as a constant-tension nanospring with defined by enthalpic balance of the phases. Progress in the synthesis of boron nanostructures suggests that the predicted unusual behaviors of 1D boron may find powerful applications in nanoscale electronics and/or mechanical devices.