Some of my recent finds from Seaham (NE England), the black glass all show different colours when a light is shone through, it's sometimes known as secret glass or pirate glass! :
A Beachcombers View - Pirate Glass | Island Free Press
Black Pirate Sea Glass Color | Rock & Gem Magazine
Large Black Sea Glass Curio Pirate Glass Vintage Black Beach Glass Shard Possibly From a Glass Tile - Etsy UK
Black Sea Glass or Pirate Glass | Costa Sea Glass
Unique Bottle Steampunk Bottle Hand Decorated Bottle Pirates | Etsy | Pirate decor, Custom bottles, Glass bottles art
The beauty of pirate glass, it looks like any other rock on the beach until light shines through it 💚🌊 www.etsy.com/uk/shop/libandleojewellery | By Lib and Leo JewelleryFacebook
Black Sea Glass Pirate Glass Vintage Beach Glass Pieces From Antique Bottles in a Shade of Green so Dark They Appear Black - Etsy Australia
What is Pirate Glass? - Discover The Outdoors
THE TRUE COLORS OF BLACK SEA GLASS – Echoes of LBI
Pirate sea glass, rare black sea glass from Wembury Beach Devon jewellery making/crafts
Black Sea Glass
The Truth about Pirate Glass – Beachcombing Magazine
Scottish Mudlarking - Pirate sea glass! Usually referring to thick glass that looks black but is actually green or brown when held against the light. The Spanish Armada did come up beyond
The Truth about Pirate Glass – Beachcombing Magazine
Pirate Glass. Found today at Point Lonsdale, Victoria, Australia. Anyone want to identify it? : r/seaglass
The Truth about Pirate Glass – Beachcombing Magazine
Beachcomber's View: Science of Sea Glass | Coastal Review
Beachcomber's View: Science of Sea Glass | Coastal Review