Consider the following simple example. Dust rates 7 on the Mohs hardness scale. Yes, dust! The stuff that floats in the air! It has the potential to scratch a variety of surfaces. In the event that you are a daily glasses wearer like me, you've undoubtedly run into this problem: after a few cleanings, your glasses will appear slightly hazy and you'll have to blame dust and other particles for it.
The same stands true for gemstones, as well. Attempting to clean a gemstone with no proper care will almost likely result in it being scratched by dust, among other things. As a result, be careful while selecting a gemstone engagement ring or any other ring design. Your best option is to choose a gemstone that is at least 7.5/10 in grade on the Mohs hardness scale, and ideally higher. The closer to 10/10 (diamonds) your gem is, the more durable it will be on your finger.