Square Shaped Face:
Square shaped guys have strong jaw line with wide cheekbones which makes their face not suitable with delicate or small sized styles. Rectangular frames are the best option as they complement the wideness of face.
Round Shaped Face:
If you have a round face you will be lacking definition for your features so the best option would be to go for wider frames to give your cheeks a slimmer look thus giving it an outline. Another thing you should look for is to not go with round frames but with something like wayfarers to bring out a solid boundaries around the face.
Oblong Shaped Face:
For oblong shaped faces it is better to not wear any sunglasses which extended more than the widest part of their face. Round and square styles frames both are good for you but avoid selecting small size as they will make your face look smaller. Aviator styled sunglasses are a great choice for such faces.
Diamond Shaped Face:
Diamond shaped faces have a sharply defined chin, wide cheekbones and prominent forehead which make retro and rectangular styled glasses suits most. Another thing is to look for sunglasses with color fading downwards to give more attention towards the eyes rather the bottom of face.