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Hi Tessa
I have squished a few, they're liquid inside, so probably slug eggs then.
This is a reputable potting compost,so disappointed if that's the case.
Would that be vermiculite? Anyway, I see no picture so can't comment on what it could be.
Nah, vermiculite isn't little round balls is it, it's like tiny minute chippings and flakes more than balls. Also it wouldn't squash full of liquid - mine never has anyway.
Could be fertiliser, but snail eggs look the same as cinnamon girl is describing too.