Tex. Tax Code § 23.17

An interest in a mineral that may be removed by surface mining or quarrying from a deposit and that is not being produced is appraised at the price for which the interest would sell while the mineral is in place and not being produced. The appraised value is determined by applying a per acre value to the number of acres covered by the interest. The aggregate of the appraised value of the interest and the appraised value of all other interests that if not under separate ownership would constitute a fee simple estate in real property may not exceed the appraised value that would be placed on the fee estate if the interest in minerals were not owned separately.