The concept of corner-sharing frameworks is crucial in designing oxide solid-state electrolytes (SSEs) with superionic conductivity for alkali-metal ions like Li+ and Na+. These frameworks consist of a highly covalent skeleton of corner-sharing polyhedra, which facilitates the diffusion of alkali-metal ions through interconnected, metastable interstitial sites within the structure. This study focuses on using a class of earth-abundant rock silicates as SSEs for Na/K-metal batteries. Through a combined approach of in-silico design and electrochemical characterization, we explored the relationship between structural features—such as the migration energy barriers for Na+ and K+, bottleneck pathways in the skeleton structure, polyhedron packing ratio, and continuous symmetry measure—and SSE performance indicators like ionic conductivity and phase stability under ambient conditions. Our preliminary findings suggest that a high Continuous Symmetry Measure value in Na/K-polyhedra and a low packing ratio of the skeleton structure are essential for achieving fast ionic transport for Na+ and K+. Experimental results show that applying this hypothesis can achieve Na+/K+ ionic conductivity levels between 10-0.1 mS/cm at 50 °C, using a composition entirely made of earth-abundant elements, without depending on rare-earth or multivalent transition metal ions. An SSE based on the Na-Mg-Al-Ca-Si-O oxide system was fabricated into thin, self-standing tape-cast layers under ambient conditions. These thin, self-standing layers, in a symmetrical cell configuration of Na/SSE/Na, cycled for over 50 cycles at current densities up to 1 mA/cm2 at 50 °C. The same layers were also used as SSE in hybrid (hybrid organic-inorganic composite as cathode layer) and semi-solid (porous carbon sulfur composite and liquid catholyte as cathode layer) Na-S battery cells. At 40 °C and a C/10 rate, these cells demonstrated high discharge capacities of 1000 mAh/g and 800 mAh/g, respectively. The use of earth-abundant rock silicates as SSEs, as demonstrated through a synergistic approach combining in-silico design and experimental characterization, presents a promising path for developing high-performance and sustainable solid-state batteries, with significant potential for enhanced ionic conductivity and phase stability without the need for rare-earth or multivalent transition metal ions.