Alumina exists in several crystalline forms, with the most thermodynamically stable being alpha-alumina. The arrangement of atoms in the crystal lattice contributes to the material's overall rigidity and susceptibility to fracture. When subjected to external forces, the ceramic is more likely to break than deform, unlike materials with more ductile behavior.

Grain size, sintering conditions, and the presence of impurities are a few variables that can affect the mechanical behavior of alumina ceramics. Fine-grained ceramics often exhibit improved toughness compared to their coarse-grained counterparts. Advanced processing methods, like hot isostatic pressing and pressureless sintering, can also be used to improve alumina ceramics' mechanical properties and make them less fragile.

In conclusion, alumina ceramic is indeed considered brittle due to its limited ability to deform before fracturing. This brittleness is a trade-off for the material's impressive hardness, wear resistance, and thermal properties. While it may pose challenges in certain applications, the unique combination of properties makes alumina ceramics indispensable in various industrial and technological fields.