Modern Baseball Composition
Baseballs are composed of several layers. The core of a baseball which is sometimes referred to as the “pill” is made of a ball of cork enclosed in several layers of rubber. This small cork ball is wrapped in a few layers of yarn. Then a rubber cement coating is applied and the entire ball is wrapped in cowhide. At this point the red stitching is applied and the baseball is complete.
So baseballs are made of: a small cork ball, yarn, rubber cement, cowhide, and stitching.
Historical Baseball Composition
Historically baseballs were made from a wide variety of materials and came in different weights, shapes and sizes. Early baseballs had a core that was made from melted shoes which was then wrapped in yarn and leather. In 1910 the first cork core balls were introduced .It wasn’t until 1934 that the baseball was standardized in professional leagues. They used a ball very similar to modern baseballs though it was wrapped in horsehide. In 1974 they switched the standard baseball from horsehide to cowhide.
Sources:
https://en.wikipedia.org/wiki/Baseball_(ball)
https://www.wonderopolis.org/wonder/what-are-baseballs-made-of