Stoneware is safe to use in the microwave as long as the stoneware has no metallic edges or finishes to it.
If the stoneware you has metallic edges or metallic finishes then it's not microwave safe because the microwave will cause the metallic edges and metallic finishes on the stoneware to spark and could damage the microwave and possibly start a fire.
But as long as it's just plain stoneware or any other ceramics without the metallic edges or finishes the stoneware as well as other ceramic is safe to microwave.
I've microwaved stoneware as well ceramic plates and ceramic coffee mugs nearly all my life and it's never caused any issues.