Scratchproof Lenses on the Horizon?

TechRadar has posted an article on scratch tests done on the new Apple sapphire display, including a YouTube video with the results of using sandpaper on the display.





The display resisted the garnet sandpaper better than the Gorilla Glass display, but still suffered scratches.

Time for a little science:

Garnet is actually a term used in conjunction with a family of related minerals.  These are silicate minerals (silicon and oxygen) that contain additional minerals.  Those minerals can be iron, aluminum, magnesium, manganese and/or calcium.  Hardness for garnet ranges from 6.5-7.5 depending on which of these other elements are present and their proportions.

Typical glass has a hardness of around 6.5 on the Moh's scale.  It will be scratched by anything with a higher hardness rating.  This includes the garnet used in the "sandpaper."  It also includes quartz, which has a hardness of 7.0.  (Quartz also happens to be the major component of sand and is usually the hardest element present.)


Judging from the video, Apple's sapphire display seems to resist being scratched by the majority of the garnet pieces embedded in the sandpaper.  It is only the pieces on the upper range of the hardness scale for garnet that are actually able to scratch the glass.

This suggest a hardness for the glass just over 7.0.

Since quartz is the mineral primarily responsible for causing scratching to exposed glass, having a hardness just over 7.0 would render the display virtually scratch-proof.  (Sand can contain trace amounts of harder minerals, including garnet pieces with hardness on the high-end for that mineral.  These would still scratch the glass, but they represent such a small presence in dust that scratches would be very rare.)


Finding a way to use the glass in camera lenses would basically render them scratch-proof as well.  The easiest approach would probably entail using a piece of the sapphire glass as a non-lens end element.


As a side note: the TouchID sensor appears to be covered by an actually piece of sapphire as it resisted being scratched by the harder emery sandpaper in addition to resisting the garnet.  Sapphire is the gem form of aluminum oxide or corundum, and has a hardness of 9.0.