That enormous Glim-Shard
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Your zailors have reported an enormous shard of glim on the island. "The size of a small child" they say.
Game Instructions: This will gain you an item that can be traded for a Rubbery favour and additional rewards, back in London.
Carting the thing over to your ship requires the efforts of several strong-backed zailors, and takes too long to allow for collecting smaller glim-shards. But this is a truly exceptional prize.