geeks_15

This is kind of difficult to explain and subjective, but I scratch the rock with my fingernail. If the rock is easy to scratch and make a mark on the rock it is probably a softer rock and therefore more likely to dissolve (very, very slowly - it won't disappear) in your tank and change the water chemistry. A rock that is harder to scratch is more likely to not dissolve in your tank, and therefore aquarium safe.

If you compare the scratch test with the acid/vinegar test you can get an idea of what I mean and get some practice comparing good aquarium rocks and bad ones with both tests. That way, after you have a little experience, if you find some cool rocks and don't have any vinegar on you, you can make a guess with your scratch test.