Sea glass (or beach glass) is glass foun on beaches along oceans or large lakes that has been tumbled and smoothed by the water and sand, creating small pieces of smooth, frosted glass. Sea glass originates from bottles and jars that enter the water as litter and become broken up by the waves and rocks. Green, clear, brown, and amber beach glass is common (consider the most common colors of beer-bottles), though light blues, yellows and pink varieties have also been known to appear. Much more rare are grey, purple, bright red, and the black varieties. Red and orange sea glass are found only once for every 5,000 pieces found. "Black" sea glass is rarely found and often originates from pre-1860 glass that is actually dark olive green. Some black sea glass is found around Australia, originating from 1940s beer bottles. Its rarity is due to the obscure materials that were used with glass to make the bottles, which increased its rate of decomposition.