Black glass, is not actually black but generally olive green, amber or sometimes blue, and gets its colour from the addition of iron oxide which was incorporated to protect the contents of bottles from light damage.
The use of black glass was common up until the end of Nineteenth Century, so any pieces of black sea glass will be over 100 years old.
Black glass is difficult to spot because the frosting from many years in the sea makes it look like a stone and takes a practiced eye to spot on the beach, which makes it quite rare to find in collections.
There has in recent years been a fashion to call black glass, “Pirate Glass”. This is more of a marketing ploy than being representative of the glass’s origins.
Although most of the black glass we pick up in the South of Spain has less romantic origins, the southern coastline of Spain was subject to extensive raids by Barbary Corsairs or Pirates, operating from the Barbary Coast of North Africa (Morocco, Algeria, and Tunisia) from the 16th to 19th centuries*. So there is a chance that some of this black glass is actually “Pirate Glass” after all!
*Ref: Barbary Pirates