Simply underripe black peppercorns, the brined green variety is made from fresh, rather than dried, peppercorns. You can substitute crushed green (or even black, pink, or white) dried peppercorns. Be aware that black will be quite a bit hotter and the pink and white will be milder so adjust to taste. You can use capers for the salty, briny flavor but otherwise, the taste isn't all that similar.