Understanding mineral rights

Hi trying to understand my mineral rights. My Great grandmother passed them down through thr family.

I have always been told never sell your mineral rights but recently I have been contacted with an offer that surprised me initially. So I felt it was time to get a better understanding of what I have.

In reading through the posts here i have noticed the terms gross mineral acres and net mineral acres what is the difference?

What would be a reasonanle per acre price for the mineral rights? I understand that some of that depends on the exact location but I dont have my paperwork handy.

Thanks for your help in advance.