Glass prescription lenses still offer the best visual acuity of any eyeglass lens, as they have for hundreds of years. Glass lenses have less impurities in the material compared to plastic lenses, and so they are optically clearer and easier to see through. However, over 95% of prescription lenses nowadays are made from some kind of plastic because using glass lenses has some restrictions and drawbacks. No lens material is more clear, clean, or better functioning that a eyeglass lenses made from glass. Also, because glass is a much harder material than plastic, glass lenses do not scratch as easily as plastic lenses, even if they have scratch resistant coating.
Now, here comes the bad news: Glass lenses are almost twice as heavy as plastic lenses, more expensive, and not as safe as plastic lenses. Glass lenses can only fit into eyeglass frames with a "B" measurement (vertical height) of at least 28mm, and glass lenses cannot be installed in rimless or semi rimless frames. Glass lenses take much longer than plastic lenses. You should allow two to three weeks for processing an order with glass lenses. Two weeks is the average processing time because every pair of glass lenses is “drop ball” tested before it leaves the lab. Occasionally, the lenses will fail the drop ball test, and they need to be discarded and started again. This could lead to additional delays.