The glasses are round and made of a solid plastic that is enchanted (by Hagrid) not to break in the middle. Or, at least, you had no hope until last week. So may legendary wizards and witches exist that you almost have no hope of reaching that high echelon of achievement.
Even though her wardrobe and habits are rather colorful, her predictions are usually very dark. A pair of glasses will help you focus your Inner Sight so you can make prophecies just like Trelawny.