Cold-sintered ceramics typically exhibit inferior mechanical properties compared to high-temperature sintered counterparts. We demonstrate that introducing large internal stress through highly concentrated nanodiamonds (NDs) significantly enhances cold-sintered α-quartz composites to structural ceramic levels. At 500 MPa cold-sintering pressure, uniformly dispersed NDs generate 1.2 GPa local prestress via Young's modulus difference, while pressure-modulated internal stress is evidenced by dielectric property changes. The optimized composite achieves fracture toughness of (3.65 ± 0.21) MPa·m¹/² (180% increase) and Vickers hardness of 10.6 GPa (80% increase), matching some high-temperature-sintered ceramics. Toughening arises from prestress-driven crack deflection and crack tip bridging, while hardness enhancement stems from NDs' rigid constraint and high-pressure-induced dislocations in silica matrix. Compressive strength increases by 90% and fatigue life exceeds 1000 cycles, attributed to internal stress-strengthened grain boundaries and improved toughness. This work presents a transformative strategy for developing damage-resistant ceramics, meriting further exploration of scalability and engineering applications.