It is a given that when you ride a motorcycle one should/must wear protective eye wear. In the past I purchased some very nice riding glasses only to sit on them, drop them, or worst.
I found sun glasses made with Polycarbonate Lenses are the best bang for the dollar. Some of the features are as follows:
I found these sunglass at a local motorcycle dealer at a great price. I paid $5.99 for each pair. If I sit on them, drop them, or scratch them I will not cry or moan able it. I carry a yellow pair for night riding, dark smoke for daylight, and clear for backup.
There are name brand designer sunglasses for bike riding as high as $200.00 a pair. Buy what you want or what you can afford. I have a small wallet and always try to get the most bang for the dollar..