You get them from hitting rocks or blouders with your hammer. You can use them to build houses or Barns or sheds. You can also hire sprites to clear the areas which collect Material Stones. Materail Stones can be used as fences to. But Unlike lumber they don't break as often.
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You can find the moon stone in Harvest Moon Magical melody in the Cave. (I don't believe that their are moon stones in the underground lake, but hey it is wroth a shot.)
Stone
Well you can't get a sun stone in harvest moon island happiness. In Harvest moon sunshine island you can get sun stones but u find them in hidden places like the water wheel on your farm. Hope that helped : )
the third mine it is randomly shuffled in one of the stones on each floor
Go to the site posted to see how to get all the Sunstones.