Worked on the leanprover-community/mathlib4 repository to formalize a key result in Euclidean geometry, proving that the diameter of a Euclidean ball is twice its radius. This contribution included supporting lemmas for spheres and closed balls, with all proofs written in Lean using formal verification techniques and mathematical rigor. The work involved refactoring and restructuring existing proofs in the geometry stack to improve clarity, efficiency, and maintainability for future developments. Collaborative authorship facilitated knowledge transfer and code hygiene, while the changes established reusable geometric lemmas that enhance downstream proofs. No user-facing bugs were addressed, focusing instead on foundational formalization.