LinePick...argh

You mean, if no mesh is picked, you don't want to place any entities, right?
Then I think this code would do that:

LinePick ccx, 20, ccz, 0,-9000,0
If PickedEntity() = terrain Then
  cccopy = CopyEntity( gem )
  PositionEntity cccopy, ccx,PickedY()+1.5, ccz
End If