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Does anytime know what this is, found about 10cm below the top soil in our garden, which is in a former coal mining area (I'm pretty sure we're on top of either the spoil heap or material taken from it)?
Rudimentary experiments show that it weighs about 1.8g per ml of volume. It's hard, black, shiny, with sharp edges.
Magic healing crystal? Or lump of coal?
Looks like slag to me. i.e. the cooled and hardened residue of whatever was burning / melting
If I didn’t know about the coal mining I’d say something like obsidian, which is pretty cool-basically super cooled lava. The mining makes me think otherwise though.
Is the slag an output from the mining itself? I always thought it was something left over from furnaces?
Something about it doesn't look quite right for obsidian, maybe the air bubbles, compared to the bits I collected as a child.
Definitely hard? As it looks like tar to me.
Some has broken off in video?
Its almost always slag sadly, albeit some of it can be pretty colourful and cool.
Definitely hard? As it looks like tar to me.
Some has broken off in video?
Yep, very hard, but possibly also quite brittle on the edges, but I was going at the ground with a pickaxe so it could have just got a whack from that.
Edit: I can't mark it with a stanley knife blade.
Basic Slag!
By product from iron making. Common anywhere you find coal in the UK as geologically the carboniferous coal measures are usually found in close proximity to iron ore deposits and limestone: all the basics needed for smelting iron.