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let’s say I want to show my users the exact size of different Diamond sizes.
Won’t the sizes be different depending if a user is on a desktop computer versus iPads and even versus an IPhone ? I am just not sure how to keep the dimensions the same so a user can actually see how big different Diamond sizes are in real life.

If you mean have it exact size on the screen, you just can’t.
I have 3 computers here, all three are Windows, doing 1920×1080 output… One is a 13.3″ screen, one is 15.6″ screen, one is 24″ monitor… So same resolution, 3 different sizes.
The best you can do is give a known reference item (or rule) beside it.