Ultra-massive hydrogen-rich white dwarfs (WDs) are expected to harbor oxygen/neon cores resulting from semi-degenerate carbon burning when the progenitor star evolves through the super asymptotic giant branch (SAGB) phase. These stars are expected to be crystallized by the time they reach the ZZ Ceti domain. We show that crystallization leads to a phase separation of oxygen and neon in the core of ultra-massive WDs, which impacts markedly the pulsational properties, thus offering a unique opportunity to study the processes of crystallization and to infer the core chemical composition in WD stars.