Tiny little hard things in cooked ground beef?

I just made some ground beef and in pretty much every chunk there is a tiny very very hard white thing in the middle. I don't know what it is. It is so hard it could break your teeth. I thawed the meat in some hot water in the sink (I do this all the time, takes about an hour). It has never done this before. Is it safe? What the #$#% is it?