Hello and welcome to the forum. As Tarquin says, they can have bits of Pyrite on the surface. The specimen can be polished using course > fine grades of 'wet & dry' paper then a final polish. I am in the process of updating an article I wrote for this forum all about it, but the original article no longer exists I'm afraid.
This one has a whole outer coating of a gold colour and shimmer, I'm hoping to polish it but need help removing the clay first if you have any tips?