This question was asked on the question and answer website Quora. It was the first question that I attempted to answer on that site. This is what I wrote:
While I think it might be technically possible to coat a diamond in teflon, there are some issues which you would need to consider:
Jewellers and jewellery buyers are very suspicious of treatments, including coatings. While teflon is colourless, a dye could be added to enhance the colour of the diamond and make it look better than it actually is. Gemstone buyers are willing to pay a premium for natural, untreated stones.
One of the things that makes a diamond beautiful is its exceptional hardness, which allows it to take a very high polish. A coating would not be able to take such a high polish and would be less attractive.
Teflon is softer than diamond, so it would easily rub off or get scratched away from the diamond surface. Again, this would make the surface appear less highly polished and therefore less attractive.
The refractive index of teflon is very different to that of diamond, it is much lower. But it is somewhat higher than air. This means light entering and leaving the diamond will refract differently to how it does if it enters the diamond directly. This means the diamond would need to be cut with the facets at different angles to the way they currently are, which is to maximise the amount of light reflected from the stone and the play of colour. This would not be a difficult thing to do, but it commits the stone to being a teflon coated stone. If the owner changed their mind and decided to remove the teflon, the stone would need to be recut to maximise its brilliance.
Since I answered this question on 22 December 2018, it has attracted 46 views and no upvotes. That is a bit disappointing as I think I did a good job. But the question is quite obscure, I can’t imagine too many people having sleepless nights wondering if they could put teflon on a diamond’s surface.
I think I chose to answer that question for my first attempt because it would not attract too much attention and I could expect to have it all to myself. So, just a first toe in the water.