Etch pits represents the symmetry of the crystal.From the shape of the etch pits you can get the crystal symmetry along the direction perpendicular to the surface. Again by counting the number etch pits per square cm( in the magnified image) you can calculate the dislocation density along that face.Etch pits occur at the crossing point of two dislocation line.These points are highly energetic.For this reason these portion of the crystal get dissolved at faster rate.