Sea glass

In the picture above, there is genuine beach sea glass on the left and on the right of the picture, artificially tumbled craft glass. This photo shows the artificial glass covered up.

Sea glass:

Sea glass (also known as beach glass, mermaid's tears, lucky tears, and many other names) is glass found 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 is one of the very few cases of a valuable item being created from the actions of the environment on man-made litter.

Sea Glass: The Search on the Shore (it's harder to find these days)
North American Sea Glass Association