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 gray, purple, bright red, and the black varieties. Red sea glass is found only once for every 5,000 pieces found, while the rarest of all seaglass, orange, can be found about once every 10,000 pieces. "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.