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It depends on the Thurry itself and obviously the size! I have a 1 x 4'' Thurry that I do about 50 passes on whereas my Escher I may do less.

I have heard Lynn say somewhere around 20 passes on...

This is a brown (ish) Thuringian with a similar pattern. I believe yours is a blue variety.

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Judging by its looks and your opinion that it does not harm your coti's edge, Id say it was a stone of Thuringian origin. I have seen pictures of similar ones to that with the array of colours.
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