If your grandmother hold the mineral rights via a deed to a real property to the exclusion of the owner of the real property, she owns the mineral rights free and clear. The threats to sue her for the mineral rights have no legal basis and can be viewed as intimidation.The attorney threatening to sue for such mineral rights where your grandmother presumably has a deed to such rights is bordering upon an ethical issue that could involve the Ohio State Bar in the threats of a lawsuit if the mineral rights are not sold create a question of improper conduct by the attorney under long established ethical standard applicable to attorneys across this country.A value can be placed on a deed to a property's mineral rights. Typically you would need to retain a geologist and an appraiser to do this. I recommend that your grandmother immediately retain a competent real estate attorney experienced in mineral rights.