AbstractMore than 3900 crystalline borates, including borate minerals and synthetic inorganic borates, in addition to a wealth of industrially-important boron-containing glasses, have been discovered and characterized. Of these compounds, 99.9 % contain only the traditional triangular BO₃ and tetrahedral BO₄ units, which polymerize into superstructural motifs. Herein, a mixed metal K₅Ba₂(B₁₀O₁₇)₂(BO₂) with linear BO₂ structural units was obtained, pushing the boundaries of structural diversity and providing a direct strategy toward the maximum thresholds of birefringence for optical materials design. ¹¹B solid-state nuclear magnetic resonance (NMR) is a ubiquitous tool in the study of glasses and optical materials; here, density functional theory-based NMR crystallography guided the direct characterization of BO₂ structural units. The full anisotropic shift and quadrupolar tensors of linear BO₂ were extracted from K₅Ba₂(B₁₀O₁₇)₂(BO₂) containing BO₂, BO₃, and BO₄ and serve as guides to the identification of this powerful moiety in future and, potentially, previously-characterized borate minerals, ceramics, and glasses.