The word polo is derived from the Tibetan word “pulu” which means ball.
A polo ball must be within the limits of 3 to 3-1/2 inches in diameter and 3-1/2 to 4-1/2 ounces in weight. A polo ball is usually made of a solid hard plastic. The most common polo balls in use are:
TEC Smooth Polo Ball
Texas Polo Ball
Polo Gear Argentine Polo Ball
In the past, polo balls were made of wood. Wooden polo balls made a whistling sound as they moved through the air. Because of this, polo players could actually hear a ball coming and thus avoid being hit. However, wooden polo balls very often split apart when hit.
Modern plastic polo balls do not produce a whistling sound but they very rarely split apart. Bernard Cohen invented the modern day plastic polo ball in the 1970’s. After testing was completed at Palm Beach Polo Club in Wellington, Florida, Bill Ylvisaker was so pleased with Bernie’s new plastic ball that he switched the club from wooden polo balls to plastic and never looked back. The new ball had an historic impact on the level of play and changed the game dramatically. Bernie named his new company TEC (an acronym for his lovely wife Trudy Elizabeth Cohen) and began manufacturing and selling the new polo ball with Tony Coppola.
A polo ball becomes misshapen every time it is struck by a polo mallet. Umpires and flagmen replace polo balls as necessary during a game. Each polo match requires approximately two dozen polo balls. The mounted umpires carry a pick-up stick to retrieve a polo ball from the field without having to dismount. This saves time during the game.
Arena Polo Ball
The polo ball used for Arena polo must be not less than 12.5 inches or more than 15 inches in circumference.
The weight of an indoor Arena polo ball must be not less than 170 grams or more than 182 grams.
In a bounce test from 9 feet on concrete at 70°F, the rebound should be a minimum of 54 inches and a maximum of 64 inches at the inflation rate specified by the manufacturer. This provides for a hard and lively ball.
http://www.sportpolo.com/equipment/Ball.htm