So yep - the reason I said I didn't think the green one looked like a Thuri was that it looked to have a few dots or speckles on it. Thuris can have slight surface patterns but they're largely very consistent, and it wouldn't really be dots like that.
The big 10" green one you might have seen of mine a few pages back is a good example of how uniform even the very large ones tend to be. Though this can make them annoyingly difficult to ID just from pictures, unless they have the 'typical Thuri saw marks'. They are though very easy to tell in person if you know how they feel, as they're incredibly distinctive to the touch and in use. The other reason I said I didn't think it was a Thuri was the size - they tended to get cut quite small, and pretty much never over 2" wide afaik, whereas your stone looks larger...? Having said all that I'm no particular expert on Thuris (or anything else for that matter), so don't take anything I say as gospel!
You're right that any size of labelled Escher&Co. Thuringian goes for silly money usually, and it's a bit difficult to find them on the cheap as people can just google the name on the label and see it's worth hundreds. Though you can sometimes get lucky with the type that I posted above because it only says 'E&Co.' in tiny letters inside the traditional cup logo, so sometimes people might miss that. I'll ping you a message as I do have a couple of Thuris, including a smaller 5x1 Escher, that I could let go if you want to try one. Maybe swap for something. If you're mostly honing razors then you definitely want to get your hands on a Thuri at some point - they're really, really good.
On the subject of not being an expert about anything... when I said the pretty patterned stone of yours was a soft ark - that was to say that it's not one of the old Pike-Norton Washitas, whereas a couple of your other stones almost certainly are. It could well be a Washita produced by another company. And as you've probably seen in this thread some people make a distinction. I've never tried any kind of soft ark, or one of these other Washitas, just the old P-N ones, so really can't say how they compare. And that's not out of any kind of predjudice btw, just a quirk of where I am; most of my old stones are from the UK which as a market massively took to the original kind of Washita and the name had a lot of cache, plus for most of the 20th century Pike-Norton had a subsidary company and production factory in the UK. Which basically means that old Washitas are incredibly common there and can be found for very little money, though you rarely see soft arks or stones from other companies. In fact when an old soft ark came up on ebay a few days ago for $40 I jumped on it, even though that's more than I've paid for most of the old Washitas I've found. As with anything - scarcity undoubtedly plays into the perceived desirability and price of stuff, so I'm loooking forward to seeing how it compares to my Washitas. And interested to hear if you think there's much of a noticeable difference...
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SG is another quite big topic. In general though as I understand it - if you know that what you've got is a type of American novaculite, you should be able to then use it to make a distinction between Soft/Hard/Translucent&Black. As your SG goes up toward about 2.65 the stones get finer.
The old style of Washitas are a little more complicated because they can have quite a wide range of SGs - anything from about 2 to about 2.5 (my lightest is 2.08, and my heaviest 2.49 I think). Which means that you can't really use it to distinguish a Washita from an 'Arkansas' stone, as soft and hard arks run throughout this range. The most common Washita weights that I've come across, and would consider most typical are in the middle of that: 2.25 - 2.35. Again heavier stones will be finer and usually more translucent, though all will have some small translucency at the edge. Though it doesn't have so much of a bearing on the speed of the stone; you can get quite fine but very fast stones.
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As you say - it's all very confusing! One thing you can be certain of though is that you've got a pretty sweet collection there
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