Croquet is a sport that involves hitting plastic or wooden balls with a mallet through hoops embedded in a grass playing court.
Like other sports there are standards for the ball equipment that is used in this sport
The Balls
Croquet balls shall be colored respectively blue, red, black, and yellow. Balls to be used in USCA Titled National, Regional, Sectional, District, or State events shall have been approved by the World Croquet Federation. The size of these balls shall be 3 5/8 inches in diameter with a milled surface and of even weight, not less than 15¾ ounces, nor more than 16¼ ounces. For these USCA titled events, all balls shall, when dropped from a height of 60 inches onto a 2-inch steel plate imbedded in concrete, rebound to a height of not less than 30 inches nor more than 45 inches. All balls within a set must not vary in rebound by more than three inches. Faulty or damaged balls may be changed during play. For all USCA sanctioned events, the manufacturer and model of the balls to be used should be included in the entry forms distributed to players prior to the event.
References:
http://www.croquetamerica.com/croquet/rules/american/introduction.php#theballs
Photo – http://www.meadowood.com/sports/croquet
http://www.ehow.com/how_3111_play-croquet.html