Billiard ball
Russian pool and kaisa—68 mm (2 11⁄16 in)
Carom—61.5 mm (2 7⁄16 in)
American-style pool—57.15 mm (2 1⁄4 in)
British-style pool (largish) —56 mm (2 3⁄16 in)
Snooker—52.5 mm (2 1⁄15 in)
Scaled-down pool—51 mm (2 in) for portable miniature tables
A billiard ball is a small, hard ball used in cue sports, such as carom billiards, pool, and snooker. The number, type, diameter, color, and pattern of the balls differ depending upon the specific game being played. Various particular ball properties such as hardness, friction coefficient and resilience are important to accuracy.
History
Early balls were made of various materials, including wood and clay (the latter remaining in use well into the 20th century). Although affordable ox-bone balls were in common use in Europe, elephant ivory was favored since at least 1627 until the early 20th century; the earliest known written reference to ivory billiard balls is in the 1588 inventory of the Duke of Norfolk. By the mid-19th century, elephants were being slaughtered for their ivory at an alarming rate, just to keep up with the demand for high-end billiard balls – no more than eight balls could be made from a single elephant's tusks. The billiard industry realized that the supply of elephants (their primary source of ivory) was endangered, as well as dangerous to obtain (the latter an issue of notable public concern at the turn of the 19th century). Inventors were challenged to come up with an alternative material that could be manufactured, with a US$10,000 prize being offered by a New York supplier,Phelan and Collender. (This would be worth approximately $177,775 in 2016.)