an old st christopher pendant

this st christopher pendant was given to my son from his great grandmother - but since she is gone - we donnot know the age - or how to replace it - if something should happen to it - it apperas to be gold and when my son had an accident and was rushed to the hospital the hospital refused to take it off for the operation ( which is they informed me- they must take off all jewerly) - for they did not know at first what it was lol and when they asked questions lter - i explained it was from his great graandmother and we know nothing of it - except it was a st. christopher - hoping someone can tell us something about it - thank you