Some boules are made of stainless steel (inox). Some boules are made of carbon steel (acier au carbone). And some boules (especially inexpensive leisure boules) are made of carbon steel coated (revêtu) in chrome (chromé). The chrome makes the surface of the boules tougher and more resistant to rust (rouille). The problem is that the process of manufacturing chrome-plated boules uses Chromium 6, and Chromium 6 is highly toxic.