Looking at that big smashed-glass coaster from a different angle showed interesting patterns:
Although the larger fragments were still holding together when I laid them in their recesses, they apparently consist of several sub-fragments with larger continuous cracks letting the epoxy flow / ooze inside.
Now that I know what to look for, the original picture also shows them, albeit less distinctly:
They’re not obvious in the scanned image of the fragments, although I could convince myself I see some:
The many smaller fragments I’ve been turning into coasters probably separated from similar large chunks along such cracks, which is why I’ve never seen rivers of crack before.
Apologies if you arrived here expecting a tirade concerning the drug trade … :grin:
The Smell of Molten Projects in the Morning 
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