Abstract <p>Quantum coherence underpins the performance of all quantum technologies, from error-resistant operations to long-lived quantum memories and ultrasensitive sensors. Despite steady advances, coherence remains fundamentally constrained by environmental noise, most notably, from nuclear spins embedded in the host lattice. Protocol-level strategies, such as dynamical decoupling or operation at clock transitions, mitigate this noise externally but impose control overhead. In contrast, materials-based approaches—especially isotopic purification—directly suppress decoherence at its source. Here, we introduce a unified periodic table of quantum coherence that quantifies coherence limits element by element, incorporating both natural isotope distributions and practical routes for isotopic enrichment. Importantly, this framework provides, for the first time, theoretical upper bounds (<i>T</i>_2,max) for all stable elements, offering a quantitative ceiling on what isotopic purification can achieve. The table reveals where purification yields decisive improvements, where alternative strategies must prevail, and where supply or regulatory barriers constrain feasibility. Beyond guiding the search for solid-state qubit hosts, it uncovers overlooked opportunities in functional oxides and related compounds. More broadly, this framework positions isotope engineering as an increasingly important tool for identifying and realizing future quantum-coherent materials.</p> Impact statement <p>Quantum coherence is the essential resource for all quantum technologies, yet it is fundamentally constrained by nuclear spins in the host lattice for solid-state qubits. While active controls can mitigate decoherence externally, materials-based approaches—most notably isotopic purification—suppress noise at its physical origin. In this work, we introduce a unified periodic table of quantum coherence that establishes, for the first time, theoretical upper bounds on spin coherence (<i>T</i>_2,max) across the entire set of stable elements. By systematically incorporating both natural isotope distributions and realistic pathways for isotopic enrichment, this framework reveals where purification can yield decisive improvements, where alternative strategies must dominate, and where supply or regulatory barriers impose practical limits. Beyond guiding the search for solid-state spin qubit hosts, the analysis uncovers overlooked opportunities in materials and highlights new design rules for exploiting isotope engineering. More broadly, this periodic table positions isotopic purification as a universal principle for identifying, evaluating, and realizing quantum-coherent materials. The results thus have implications not only for advancing quantum information science, but also for materials synthesis, isotopic supply chains, and the integration of quantum coherence considerations into the broader practice of materials design.</p> Graphical abstract <p></p>