The international rules specify that the game is played with a sphere having a mass of 2.7 grams (0.095 oz) and a diameter of 40 millimetres (1.57 in).[21] The rules say that the ball shall bounce up 24–26 cm (9.4–10.2 in) when dropped from a height of 30.5 cm (12.0 in) onto a standard steel block thereby having a coefficient of restitution of 0.89 to 0.92. The 40 mm ball was introduced after the 2000 Summer Olympics.[18] However, this created some controversy as the Chinese National Team argued that this was merely to give non-Chinese players a better chance of winning since the new type of ball has a slower speed, while at that time most Chinese players were playing with fast attack and smashes. (China won all four Olympic gold medals and three silvers in 2000, and have continued to dominate.) A 40 mm table tennis ball is slower and spins less than the original 38 mm one. The ball is made of a high-bouncing air-filled celluloid or similar plastics material, coloured white or orange, with a matte finish. The choice of ball colour is made according to the table colour and its surroundings. For example, a white ball is easier to see on a green or blue table than it is on a grey table. Manufacturers often indicate the quality of the ball with a star rating system, usually from one to three, three being the highest grade. As this system is not standard across manufacturers, the only way a ball may be used in official competition is upon ITTF approval.[21] The ITTF approval can be seen printed on the ball.