The cue ball and object balls should behave as one matched set. A mass difference changes energy transfer, cushion rebound, and the speed needed for a shot, so random replacement balls are a poor choice for serious practice.
Verify the set
Compare diameter and weight with the rules for your discipline. If a specification is unclear, use accurate scales and inspect the finish: chips and uneven polish can disturb roll as much as mass variation.
Replacing only the cue ball
A spare cue ball should match the set’s diameter and material. A lighter or heavier ball changes break and position-play feedback. Youth and training tables may use different formats, but label them clearly instead of treating them as tournament equipment.
Keep the balls together and retain the manufacturer specification for future replacements.