I have an original Apple Watch since release day and still wear it everyday. The back is very scratched up. Not deep so I guess everyone is calling them micro abrasions but it’s very noticeable when looking at the back. It almost looks like a haze across the back. I haven’t noticed any problems with the sensors readings. I work Road Contruction so a lot of sand, dirt, mud, and dust have made there way under the Watch. Rubbing constantly between the back and my wrists. Even when tight stuff gets under there. During the day or night I have to take the Watch off, clean the lip around the sensors and wash it under water while spinning the crown and clicking the button to get stuff out, but it still works without any problems. I also have kept it on when in the pool. I am honestly surprised at the durability of the Watch. Just slow as hell compared to the new watches. So I wouldn’t worry about it too much unless you’re actually experiencing problems with the readings. I thought the back was suppose to be sapphire on all watches but I guess not.