Unfortunately, your mother will likely need to probate her mother’s (your gram’s) estate. As you likely know, mineral rights are interests in real property and therefore have a title that is recorded just like any other real estate.
So, if the property is still in your gram’s name then it needs to be put into your mother’s name before she could sell it. There may be someone interested in purchasing from your mother without her having legal title, but they would discount the interest because they would have to clear title by some method.
Who is entitled to own the property is determined by your gram’s will if she had one. If she did not then there are statutes that determine who is an heir.
Depending upon the percentage of interest, ithe costs might exceed the value of the property.
Hope this helps.