Note: I brought this to another thread, because it's too tangential for the original one.
Further Note: I am not, by any stretch of the imagination, an expert on natural whetstones. I hope anyone who is will weigh in and correct and/or amplify what's said here.
The natural stone business...now there's a whopper! I'm assuming you mean natural waterstones as opposed Arkansas' and the like. I don't have much experience with them. But I think I'm probably going to take the plunge soon. I say that with great trepidation- after buying 30 synthetic waterstones the last thing my wallet needs to is to catch the fever over naturals.
Well, yes and no, Phaedrus.
A few things to know about naturals. First of all, at any grit level, they vary widely in hardness and durability. This is partly dependent on the type of stone, but not entirely. So you can have two stones with the same name, grit level, shape, size, and everything, but one wears and the other basically doesn't. I don't know of natural stones wearing as fast as even the slowest-wearing synthetics, but certainly the average natural aoto does wear noticeably. That means the whole routine --- grind, ha ha --- of flattening and whatever, at least potentially. Further, grit levels are always approximate, and hardness and durability too can only be judged through use or by an expert. Flaws in a stone are sometimes obvious and sometimes not, and in some cases they matter a lot and in some cases they don't.
The result is that any stone can only be effectively rated by an expert. It's no good looking and seeing, "hey, this is a natural aoto, size X, so that's the thing for me." It could be fabulous and it could be cr*p. Even if you are an expert, you can't do more than make an educated guess based on a grainy photograph on line --- you need to handle the stone, look closely, see what it does in water, grind on it a bit, and so on.
So you should never, ever buy a natural stone from anyone you don't have excellent reason to trust. If the seller is a Japanese professional whetstone retailer, there are ways and means to make sure he's legit, such as membership in one of the whetstone guilds, but most of these people do not do a lot of overseas business nor work in languages other than Japanese. There are certainly exceptions, of course.
Assuming you're dealing with a reputable dealer, to a great degree you will get what you pay for. But even there you've got the problem that not every purchaser is looking for the same qualities in a stone. Sword-polishers, for example, are interested in somewhat different things than are kitchen knife sharpeners. The best thing would be to consult with the dealer about your needs.
That said, a good natural stone can be an excellent investment if you take the time and care to make a good purchase.
In particular, if you get a hard, durable stone, of a sufficient size to sharpen effectively, and an appropriate nagura or koppa to develop an initial slurry, you have probably just eliminated ever needing another stone at that approximate grit level. This is because if the stone is of good quality and fairly hard, it simply won't wear out. I have a very hard Asagi stone, somewhere in the range of 15k JIS, that the previous owner used every single day for about 40 years and never succeeded in wearing to any significant degree.
So, Phaedrus, if you have some 30 synthetics, consider what this whole collection cost you. Ignore the coarse stones for a minute --- they're cheap enough. Now let's suppose you bought 3 naturals, rating somewhere around 1k, 6k-8k, and 15k. Say you spent an average of $200 apiece (that's a lot for the 1k, cheap for the 15k, so call it even). Now suppose you never buy another stone at these levels. Are you saving money? Yes --- in the long run. Probably you'd save money even if the set of three cost $1000. But what about the short run?
And of course, before you drop $400 on that beautiful natural polishing stone, are you sure that this dealer is reputable and knows what he's doing --- and understands what you want and what you're going to do with it?