I’ve been climbing for over a decade and I was always taught that if I dropped anyone’s gear from any height whatsoever, I owed them a new piece of that gear because I may have caused “microfractures”, and the gear was no longer considered safe. For some instinctive reason, I felt like given the intense amount of forces that climbing gear is meant to withstand, it seemed really silly that they would be so fragile that dropping a carabiner, or any other piece of metal gear from waist-height could render it entirely unsafe.
The idea behind microfractures was that dropping metal gear onto a hard surface could cause these tiny weaknesses that are invisible to the naked eye. Those weaknesses could then cause the gear to fail at much lower forces, rendering them potentially very unsafe. The story then proceeded that because you couldn’t see them with your eyes, and essentially needed an x-ray to determine whether or not the gear had developed a microfracture, you were far safer just retiring your brand-spanking new piece of gear.
Now, in general, microfractures are definitely a real thing. Most climbing gear though is made from aluminum and that particular metal is far more malleable than other metals. This allows the gear to withstand far more impact that if it were made from carbon steel or some other far harder metal.
If you read through the scientific literature (which I’ve done a little bit of), you’ll see that time after time, test after test, carabiners and other metal climbing hardware do not become much weaker after being dropped.
Anecdotally, a friend of a mentor of mine who works for a gear manufacturer (and has access to very cool testing equipment) went to the base of El Cap after a climbing season and gathered all the cams, biners, nuts, etc… that were dropped from unknown heights on El Capitan. He then put each of these pieces in a pull-test machine. He found that as long as there were no visible signs of damage, there typically weren’t any significant decreases in strength. When there were visible signs of damage however, then there were decreases in strength. That said, if you at any point in time are concerned about the safety of the gear, it’s best to retire it promptly. The last thing you need is to be worrying about whether or not the piece of pro that’s protecting you from hitting the ground is going to hold a fall when you’re in the midst of pulling the crux.
Moral of the story is this: If it looks good, it’s probably good. If it looks bad, it probably is. If you’re ever worried about it, feel free to donate it to me.