Chemical exposure in normal household settings (chlorine bleach, lemon juice, ammonia), diamond on sapphire, sapphire on sapphire etc. Just chlorine exposure over time (swimming, doing household cleaning) can actually weaken sapphire to the point of disintegrating.

Remember, precious and semi-precious stones are just fancy chemical compounds.

I have a tungsten carbide wedding ring, which also rates 9 on the Mohs scale. It's totally scratched up around the outside, and shiny around the inside, after ten years. And I'm careful to take the ring off before I use abrasive materials such as sandpaper and diamond sharpening tools.

You don't know her friends though. They might wear all rings covered with diamonds.

I think at the point where everybody's wearing rings with set stones, they mostly stop fist bumping.