How does cartilage become bone?
The process is called endochondral ossification.

It is important to point out that not all bone is derived from cartilage. For example, when a bone is fractured and is healing, the new bone does not always come from cartilage, rather intramembranous ossification occurs. There are therefore multiple mechanisms at a microscopic level by which bone may form. Additionally, bone formation in the long bones is different to that in, for example, the flat bones of the skull.