What kind of damage could this do?

If you're going to mow, not a bad idea to first stroll around and pick up debris that the mower might hit.

The object you've described is not common, so I'm left wondering what it was and how it got there. In my yard, a block from a high school, it's soda cans and food packaging, plus my own tree's pine cones. (They won't hurt the mower, but dull the blade and scatter pieces in the yard.) Chances are that you'll just want to have the blade sharpened. If it was really bad (like hitting a pave stone), it would have stopped the mower, bent or broken the blade, and/or damaged the mower itself.

If you didn't notice any vibration after the metal was struck, then the mower blade shaft is probably okay.

Your blade is most likely nicked. It will probably need to be removed, straightened, and sharpened.

A good rule of thumb is to walk the area with a bag before you start mowing. Pick up rocks, bottles, cans, and other debris that could cause injury or damage your mower. An ounce of prevention. . .

The worst would have been broken the blade and have it hit your leg or bend the shaft it`s mounted on.. As long as you have NO vibration the shaft should be ok but to be safe, I would get a new blade. If the old is bent it will wear on the shaft bearings then your in trouble,. Good Luck.

it can dull your blades dent or even chip them it can get stuck and jam the blades then you will hear a squealing noise or you can bend your deck hope this helps

bad damage