Sea glass is glass found on beaches, which originated as pieces of glass from broken bottles, broken tableware, or even shipwrecks, which are rolled and tumbled in the ocean for years until all of their edges are rounded off, and the smooth surface of the glass has been worn to a frosted appearance. It is not just the wave action that gives the glass its unique appearance, but the chemical action of the sea water, and exposure to the air as the glass is tumbled in the surf.
The history of settlement in the area and trade in the waters off southern Spain goes back nearly three thousand years to the age of Phoenicians, and continues through the Greeks, Carthaginians, Romans, Moors, as well as the Europeans, and with its proximity to the busy Straits of Gibraltar this has led to plenty of opportunity for glass items to end up in the sea.
Another reason is the fact that up until very recently most Spaniards were oblivious to ecological concerns and were quite happy to dump their rubbish, including bottles, into ravines and dry river beds to be washed out to sea with the winter rains.
Whatever the reasons it means a treasure trove for modern day beachcombers and glass pickers.