Beaters having difficulties with ball-shaped Bludgers made from rocks shattering into gravel, they first try using lead Bludgers, which prove to be too soft. They eventually progressed to making the balls out of iron – the material still used today (QA6).
From the Web
Pottermore feature: Quidditch collection
Quidditch lists on the Lexicon: Quidditch
Harry Potter Wiki: Quidditch