Mud

MLB Rule 3.01, part C, states that:

Before the game begins the umpire shall:
(c) Receive from the home club a supply of regulation baseballs, the number and make to be certified to the home club by the league president. The umpire shall inspect the baseballs and ensure they are regulation baseballs and that they are properly rubbed so that the gloss is removed. The umpire shall be the sole judge of the fitness of the balls to be used in the game;

What exactly is used to remove the gloss from baseballs? MUD! And here's where it gets a little bit more interesting. It's not just any mud, it's "special mud."

The mud is from Lena Blackburne Baseball Rubbing Mud. The exact location of where the mud comes from is supposed to be  secret, but an internet search reveals its mud from Rancocas Creek, on the New Jersey side. There is even a google maps link out there to help narrow it down even further.

The mud isn't just packed at the site and sent to your local ballpark. No, it is cleaned, screened, and baking soda is added as a "secret ingredient."  

Each year, the company visits the site and stores away 1,000 pounds for the season. It is considered to be the "perfect baseball-rubbing mud."