I thought it could be, which would be nice, but then decided maybe more likely a Grecian / Llyn Idwal (a greenish colour is maybe more noticeable in a couple of the other pics). And certainly thought the broken one looked like a Trans Ark as @rideon66 said.The speckled one in the middle is probably a Tam O’Shanter.
And then after posting this I noticed that the seller had a number of other old stones listed, where he had identified the type of stone. So I asked him what he thought... Probably Translucent Ark, Grecian Stone, Welsh Purple Slate (I assume this is the one on the right), and an unknown stone (I assume the small, dark one that also looks like a slate).
I'm quite happy with those, £35 for the lot. You'd be welcome to have one of the Ark halves if you want (?) but it sounds from what you said above that you might have a similarly small sized translucent already.