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If he payed Jnat price for a Cnat, the level of great might be slightly diminished

I think may be it is a good idea to contact the seller and ask him about the origins of the stone.
Also some better pics of the surface may help determine if the stone is a Jnat or Cnat or...

The thing is the Chinese naturals vary in appearance, there are striped ones , with brown bands, etc. Glen made a thread but I can't remember the title.
Here a few threads I was able to find...

For me it is hard to tell, if you have a Chinese Natural to compare to, and it feels different, then most likely your stone is not a Chinese natural.

Hard to tell from the pics.
Can you do a close up of the surface, and the side?

Right now ti looks like the Chinese natural