The ball shall be a sphere formed by yarn wound around a small core of cork, rubber or similar material, covered with two strips of white horsehide or cowhide, tightly stitched together. It shall weigh not less than five nor more than 5¼ ounces avoirdupois and measure not less than nine nor more than 9¼ inches in circumference.
Tim Lincecum is displeased with baseballs kept in Colorado Rockies' humidor
The current Rawlings baseball is made with a cushioned cork "pill," a cork wrapped in rubber and wound in a series of three wool and one cotton yarn. Two pieces of cowhide are hand stitched together with 88 inches of red wax thread and 108 stitches. The seams are machine rolled for about 15 seconds to compress the stitches. Then the ball is subject to cosmetic grading, measurement and grading.
Rawlings baseballs are periodically tested for coefficient of restitution (COR). The baseball is shot out of an air cannon at a velocity of 85 feet per second against a wood surface eight feet away. The rebound speed is divided by the delivery velocity to calculate the COR. The baseballs must register a rebound 54.6% of the original velocity, plus or minus 3.2 percent. They also must retain their shape after being subjected to a 65-pound force and distort less than 8/100ths of an inch under compression.
Cowhide replaced horse hide after the 1973 season.
Each baseball used in a Major League contest is hand rubbed using Lena Blackburne's Baseball Rubbing Mud. This mud comes from a secret location along a tributary of the Delaware River. Blackburne's mud has been used by every major league team for over 50 years.
Reporter piece on rubbing baseballs before an MLB game:
Discovery / Science Channel's "How It's Made" Baseballs episode
Rule 5.02 CMT (If the ball becomes damaged during a play, it remains live until the play ends and the umpire calls time)