Brittle Material deforms by fracturing. Glass is brittle. Rocks are typically brittle at low temperatures and pressures. Ductile Material deforms without breaking. Metals are ductile.
Many materials show both types of behavior.
They may deform in a ductile manner if deformed slowly, but fracture if deformed too quickly or too much. Rocks are typically ductile at high temperatures or pressures. Viscous Materials that deform steadily under stress. Purely viscous materials like liquids deform under even the smallest stress. Rocks may behave like viscous materials under high temperature and pressure.