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Not glass, but hardened resin. Used to track littoral currents and drift. Has metal so it can be found later with a metal detector.
A tracer pebble, interesting. But they seem to be rather made of the same pebble they are supposed to trace. https://southerncoastalgroup-scopac.org.uk/tracer-pebble-studies/
Fascinating!
Winner. 2 hours!
![[Challenge-Solved] glass egg thing](https://lemmy.world/pictrs/image/54d923bf-4a32-46aa-8ece-9ac9bf5e614e.webp){kind=link}