Rare-earth cations have up to six-hour hyperfine energy level coherence times and several millisecond optical coherence times because of 4f-4f orbital optical transitions well-shielded by filled 5s and 5p orbitals. The long-lived states allow for faithful transfer between optical and spin qubits. Building rare-earth qubits typically involves low dopant concentrations to improve coherence times, but along with low optical density comes site inhomogeneity and reduced efficiency. However, stoichiometric rareearth materials with drastically higher rareearth concentrations and improved site homogeneity can reduce inhomogeneous linewidths to the point that transitions involving hyperfine energy levels separated by only ∼70-140 MHz are resolved. Promisingly, EuCl 3 •6H 2 O has demonstrated an ultra-narrow 5 D 0 → 7 F 0 transition linewidth paired with a near-millisecond coherence time, but its environmental instability and needed isotopic enrichment prompt searches for additional candidates.