Single-phase in situ toughened SiAlON ceramics containing various modifying cations and single-crystal seeds were studied. The modifying cations include rare-earth cations from the smallest to the largest allowed in the α-SiAlON structure (Yb to Y, to Nd), and from monovalent to trivalent (Li to Ca, to rare earths). At low seeding levels, the aspect ratio of grains increases with the size of modifying cations, giving rise to rather different appearances of the microstructure in different SiAlONs. A one-to-one correspondence between seed crystals and large grains at low seeding levels is also observed. An optimal amount of seeds is required to maximize the fracture toughness, which is controlled by grain pullout with the fracture energy that scales with the fraction of elongated grains, their width, and their aspect ratio. The optimal amount of seeds required to reach maximal toughening increases with the aspect ratio of grains and is the lowest (1%) in Y- and Yb-SiAlONs.