"They are undoubtedly Bludgers rather than cannon balls. The faint indentations of magically reinforced Beaters' bats are visible and one can see the distinctive hallmarks of manufacture by a wizard (as opposed to a Muggle) — the smoothness of line, the perfect symmetry. A final clue was the fact that each and every one of them whizzed around my study and attempted to knock me to the floor when released from its case."
-- Agatha Chubb (QA6)
Agatha Chubb was a witch whose expertise was in ancient wizarding artefacts.
While researching the Quidditch practices of the 16th century she discovered twelve Bludgers made of lead in peat bogs and marshes in Great Britain and Ireland (QA6).
She used her finds to show how modern wizards had changed from lead, which was too soft to make a proper Bludger, to more robust iron Bludgers (QA6).
Commentary
Etymology
Agatha is the Latin form of the Greek name Αγαθη (Agathe), derived from Greek αγαθος (agathos) meaning "good" (Behind the Name).
Chubb comes from West Country English nickname deriving from the fish (chubbe in Medieval English), which has a voracious appetite and is short, fat and sluggish. The nickname was originally given to persons who fit this description (Internet Surname Database).
From the Web
WizardingWorld.com (Pottermore) feature: Quidditch Through the Ages
Harry Potter Wiki: Agatha Chubb