Yeh thats possble, althogh its a smoth rock, but it could have done that on the gravel but im not sure, and hears you pictureWould really help if you can get another picture from above, that way we can have a good look at what the skin is like around the bone.
This fish also has a recommended tank size of 100 litres your tank works out to 58. It is possibly that this fish is quite an active swimmer and may have damaged itself on the rock in some way.
The wound looks clean.
Any jaggered edges to the skin.
Any tear marks like he has caught it on something in the tank.
Dosn't look like an injury but could be wrong.
Looks like someting eaten the skin away to me.
I agree with tigerissey if they red marks the wounds needs dabbing.
I would also add the tetra med to stop septicemia setting in.
Is would be better if you could issolate the sick fish.
Do you have any methylene blue.
Wrap the fish in a cloth soaked in tank water. Ask someone to hold the fish down for you. cotton wool soak in tank then squeeze the water out. Add the lotion to cotton wool and they dab the wound all around the infected area.
Make sure you can use the tetra med raw on a fish.
You could just try some salt diluted down in water.
Once you have applied the antiseptic just allow a minute or so for it to saturate the wound before releasing the fish back into the tank.
You will probably need to do a water change everyday as keeping the water as clean as possible will aid recovery and keep the wound as clean as possible. Prob about 25% WC.