It's been longer than I thought (prolly 7 years since I bought it -__-) Couldn't find the stone in my eBay purchase history nor in my mails :/ but doing a quick "Natural 12K grit" threw results that helped me search for a similar stone.
It looks a lot like this one: Sharpening, natural whetstone 12k grit size, about 3x8 in. World's best. | eBay
The size is that of a standard sharpening stone (I bought a coarse one for other kind of jobs) and the stone does have a nice dark gray.
I can't really say it's that one but I am pretty sure it's a Chinese stone (and the Guangxi name rings a bell)

I made sure to flatten it properly (at least that's what I remember) I used the pencil technique and after a few minutes of rubbing it against a 800 grit wet sandpaper, all the lines were gone the stone kept the smoothness but this time it was flat.

I hope this info helps you help me :P I am confident that stone is trustworthy since I managed to get my hands on a decent magnifier and after some passes (back when I tried to fix the blade) I checked it with the magnifier and the bevel was really shiny and looked smooth... but the blade was not sharp enough.