How to explain this ?

I had seen some DZI from TIBET that are selling for ten to twenty thousend SIN dollars. The art grain pattern of the DZI look symmetrycial and artificial. I am curious to know as a cabochon DZI should not show such kind of art grain pattern. The shop keeper could not give me a logicial explaination when I asked, his reply was during ancient time a stone fall from the sky and blessed by the lama and only limited quantity found in the tibet. dear members do you have any idea ? Thanks Regards