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 very important to the finer points of gameplay.]clarification needed[
The earliest balls were made of wood and then later clay (the latter remaining in use well into the 20th century).]citation needed[ Although affordable ox-bone balls were in common use in Europe, ivory was favored since at least 1627 until the early 20th century;:17 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.]citation needed[ The billiard industry realized that the supply of elephants (their primary source of ivory) was endangered,]citation needed[ as well as dangerous to obtain (the latter an issue of notable public concern at the turn of the 19th century).:17 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,:17 Phelan and Collender.]citation needed[ (This would be worth $175,375 today.)
Seven-ball is a contemporary rotation pool game with rules similar to nine-ball, though it differs in two key ways: the game uses only seven object balls as implied by its name, and play is restricted to particular pockets of the table. William D. Clayton is credited with the game's invention, ca. the early 1980s.]citation needed[
At the start of the game, balls 1 through 7 are racked in a hexagonal configuration, with the 1 ball placed at the rack's apex, centered over the table's foot spot, the 7 ball placed at the rack's center, and all other balls placed clockwise (see photo top right). Immediately following the break shot, the opponent must elect three pockets along one of the table's long rails. Once that selection is made, that player may only legally pocket (pot) balls in the selected pockets, and the player who broke is automatically assigned the three pockets situated along the opposite long rail. Balls must be pocketed in rotation, starting with the One, as the object ball. Balls pocketed via combinations off of the object ball are legal. No points exist for any balls. The Seven ball must be pocketed in the called side to be a legal win. Scratches on any object ball allow the opponent ball-in-hand, but the sunk ball remains potted. Scratching on the Seven ball is a loss. Pocketing the Seven in other than the called side results in ball-in-hand to the opponent, and a spotting of the Seven.