Rare Gems

General question anyone happen to know the drop chance of rare gems or has anyone found it out yet because I feel as if hextech is screwing me over pretty hard atm ive bought over 30 chests got 1 rare gem... and yet somehow my friend is lucky enough to only have 5 rare gems from opening less than 15 idk what her exact number was tho if im right it was around 13 chests she opend