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In page Field hockey:
"Standard field hockey balls are hard spherical balls, made of solid plastic (sometimes over a cork core), and are usually white, although they can be any colour as long as they contrast with the playing surface. The balls have a diameter of 71.3–74.8 mm (2.81–2.94 in) and a mass of 156–163 g (5.5–5.7 oz). The ball is often covered with indentations to reduce aquaplaning that can cause an inconsistent ball speed on wet surfaces.[citation needed]