Actually, it is an interest in the mineral rights. I would need to connect with a mining company and I would get royalties. Some of the properties have documented cobalt deposits which are likely quite valuable. Most of the fractional interests are in areas that "might" have iron ore. It was all in a bundle. I'm thinking that I will give the mineral interests that appear to be of little value (but may be near environmentally sensitive land) to conservation groups and try to get the other fractional share owners together and approach mining companies. But will the conservation groups take the interests? What about the property taxes and carrying costs?
First, do you have a list of all the fractional share owners? If not, you will need to do some research and/or engage an attorney who specializes in tracking these down. As properties and associated mineral interests are sold/reserved/passed down thru generations, they get split up and can be very difficult track down.
Second, a mining company will need to locate the deposit and determine if it is economical feasible first before deciding whether to proceed. On one of our properties, we signed a 3-year oil & gas exploration lease with an oil company for $xxxxx for them to look for any oil & gas on the property. If anything is found and deemed to be economically feasible, we would then negotiate another lease regarding extraction, royalties, etc.
If you sign any lease, you need to be very careful and do your due diligence especially regarding access, equipment, transport, any damage and pollution and subsequently clean-up, etc. This is not DIY and you should absolutely engage a specialized attorney to handle these leases with mining companies.
My experience has been with oil & gas and not with any other types of minerals, so things may work differently with other minerals.
Unless you're donating your land to a conservation group (and don't reserve your mineral rights---meaning you transfer those rights to them as well), there's no reason why a conservation group would take your mineral rights interests alone.
Also, all property tax and carrying cost are tied to land ownership (i.e. surface rights) and not to underground mineral rights. Whoever owns the land pays the tax and any costs associated with maintaining that land. You don't pay property taxes on mineral rights (unless you extract minerals and pay taxes on the income).
Caveat---I am not an expert on this by any means, so take my comment with more than a grain of salt