SnO2 single crystals have short prismatic habits bounded by well-developed {110} and {111} faces in a pure SnO2-Cu2O flux system. When trivalent cations are added to the system, the habits drastically change to needle, acicular, or whisker forms with large aspect ratios. The addition of trivalent cations also greatly increases the nucleation rate and drastically decreases the crystal size. SEM observations and EPR;LA investigations reveal that the Hat {111} faces transform to rounded or rough {hkl} faces by the addition of trivalent cations. This roughening transition of {111} faces, keeping {110} faces unchanged, is the cause of drastic habit modification that is attributed to the breaking of the periodic bond chain in {111} faces by impurity cations.