Nevertheless gravel is obtained, under the third level of palborough's mines, etc ~ it gi_ve to the refiner in I-7. Go to the second floor and pull the lever to the refiner there, to obtain the mythril sand Item.
However the Mine Gravel is obtained, take it to the third level of Palborough Mines, and trade it to the Refiner at I-7. Go to the second floor and pull the Lever at the Refiner there, to obtain the Mythril Sand item.