I you want to harden your body, i suggest earnestly that you research the subject and techniques completely before you try. There are actual methods to condition your body and without following them and blindly hitting hard objects, you can damage your body real bad.

I agree with 'ta_kuan_dao' there are actual techniques to strengthen your body.
I do the hand strengthening technique called iron palm and appart from a little 'accident' I had recently, it works and you will still have usage of your hands in old age, unless you dont care about predicting the weather by the time you are 40!
It takes about 3 years of iron palm training and then you will have achieved al you can.
You might not have the patience for it, but as far as I m concerned, there is no rush with my martial art, and there is no rush with my conditioning.
Your body cannot be taught to break stones overnight without some serious effects on your bones in later on life. And its not the bones thatare the problem its the joint problems you will have.

My advice is:
Find a book called 'iron palm' by Brian Gray and read it and do what he says. The guy knows what he is talking about as you will realise if you read it.

Knuckle pushups (the faster you try to push yourself up, the more pressure thay take) and high veolcity loose knuckle-to-wall slaps for hands..

Bottle rolling (dragging really) on the shins, gradually increase pressure and speed and timelimit..

Palmbone, hit stuff with it hard and fast.. A tip on why u may use this insead of a knuckle strike in a self defense purpose. It's faster cause u don't need to clench your fist.
Plus u can grab your opponent fast after you've hit him

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high veolcity loose knuckle-to-wall slaps for hands

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Are you serious? Back slapping walls *WILL* damage your hands and is stupid and dangerous advice. Hitting an object harder than the object you're hitting with will cause the object your hitting with to break! Striking walls is nothing but macho BS that will cause you long term injuries!

I think gradual conditioning and medication afterwards for health, is the key. Strengthening and toughen the body.

Whats funny is the strange look on the nurses face, when them try pericing your finger tip for blood. She walked away telling the other nurses, "Callaous finger tips, I never heard of such a thing, who in the world or why would someone have callous finger tips"! They peek in the room to look at the freak.

I never thought my fingers tips were calloused. But I never tried to prierce them with a needle either.

Body toughening has two purposes, none of which is to harm you. The conditioning of the body is to reduce injuries to you and have more effective technique against an adversary. You should inquire with your teachers and be supervised at the beginning. If you do not have anyone, I would not do this type of training.

-Knuckle pushups-
My friend does Knuckle push-ups or use to. He says when he punches something his hand hurts. Do knuclke push-ups damage your hands? (P.S not trying to hijack the tred )

More on knuckle push-ups:

http://www.fightingarts.com/ubbthreads/showflat.php/Cat/0/Number/15802602/an/0/page/0#15802602

actually those nurses must not be very smart IMHO!!! reason being that callous finger tips come from many occupations such as: roofing,masonry,carpentry,playing the guitar. they all will result in very callous hands ecept the guitar which will only callous the tips of your fingers