They are basically boiled then dried yak and cow milk. I can't imagine they are harder than a nylabone.
I'm probably the one person that says give the dog a REAL bone. In all the years and all the dogs, I've only had one slab fracture where a tooth had to come out. And we never had to do a dental ever. I'm OK with the risk of them chewing hard bones or antlers (though mine don't like antlers...we have a small useless pile).