My understanding is these are solid clear ceramic, not just a coating. (The ring is metal) They are also supposed to be very nearly optically pure such that there would be no perceptible degradation of the image. Sigma claims them to be 10x the strength of chemically treated glass in which only the surface is hardened. These protectors are not cheap, but if I had a $2000 lens like the New Canon 100-400 Mark II it might be worth it.