No its not a natural pattern, its either ingrained dirt or it was etched by some sort of acid
You can use something called HCL89 to remove the ingrained dirt or the etch on the stone
You can usually get it from http://hcl89.byethost10.com or http://hcl89.atwebpages.com
It also protects the stone so that the stone is always kept new like
Take care