Ah, man. You should have told a fellow Kiwi before. I'd say you have to be careful with some stones when they first arrive. I've had some that really react the first time you use them, and then after they're fine and seem settled. I've only ever had one split like this one on me, but it was a Natsuya which can be sensitive too. I'm not sure how it works, but these stones would have been mined more than a half a century ago and then just sat around after being cut, and then somewhere at some point some guy in NZ wets them.
Even though you were careful, I'd say you have to be doubly careful at first and if possible lacquer as soon as possible. Also consider how it will dry, although I don't imagine May weather in Auckland to be dry or a problem for stressing a drying stone.
Another point is, especially at first, don't use one of those sharpening stands like you have in the picture. Have the stone on a flat surface on a towel or non-slip pad. No need for the stand, as a big medium stone is already high enough. Plus, the stand may stress the stone as pressure isn't even.
This Aizu looks a bit unusual. Basically you have two types, sesame seeds and white spots with black lotus, and you've got the latter. Both have seemed equally good to me. Yours doesn't have a consistent appearance though, and those various lines can indicate cracks. Sometimes you can hear worrying hollowness when you tap on an inconsistent stone, and you can still try this. Unfortunately, seems like you've got a lower grade specimen, but as I understand it Morihei has sold some recently for about only Y10,000 while in the past when seen other sellers have had Aizu for Y20-40,000. Maybe he got hold of a cheaper few to sell, and I've never thought Morihei prices were on the economical side either, so there has to be a reason.
Aizu are not layered stones and so wouldn't have 'suji' in the normal toxic line sense. As you later found out, 'twas a crack.
Definitely, and even before I lacquer stones I wipe them down with water
'before I lacquer' - How often do you lacquer? Thought you just got your 1st natural last month! :dontknow: Lightning Mike, you move quick. Don't tell me you already have a sizable stone collection, too.
