I saw this on the beach between high and low tides and it was not easy to spot. It is the first time that I have seen this washed up.
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Looks like one of the types that used to be harvested for production of iodine in the past.
Lat/Lng: 50.7186, -1.8524
OS grid ref: SZ105910
The tide was on the turn after having receded. This is a sandy part of the beach with some shingle.